What is the value of c in the equation y=-x^2+6x+c if the y-coordinate of the vertex is 12

Question

anonymous · Answer

i need help ):

jdoe0001 · Answer

well, let's complete the perfect square trinomial, do you know how a " perfect square trinomial" look like?

anonymous · Answer

Yes by squaring the middle number?

jdoe0001 · Answer

gimme a sec

anonymous · Answer

ok

jdoe0001 · Answer

ok, lemme instead use this other formulat => $\bf \left(-\cfrac{b}{2a}, c-\cfrac{b^2}{4a}\right)\
$

jdoe0001 · Answer

that will give you both coordinates

anonymous · Answer

the answers are-A.5  B.-5  C 3  D -3

jdoe0001 · Answer

now we know that the y-coordinate is 12, that is $\bf c-\cfrac{b^2}{4a}= 12$
we know "a" and "b", plug those in, and solve for "c" :)

jdoe0001 · Answer

I don't get any of those answers, lemme quickly check by completing the square

anonymous · Answer

But were not completing the square ethier we are doing minimum and maximum

anonymous · Answer

have you found it out yet?

jdoe0001 · Answer

hmmm, rats I just noticed the negative in the "x" :(

jdoe0001 · Answer

I was using a positive "x"

anonymous · Answer

did you get the answer by negative x?

jdoe0001 · Answer

well, dohh, is terrible, heheh, those darn dashes

jdoe0001 · Answer

anyhow, yes using the formula of the y-axis I showed up there

jdoe0001 · Answer

plug in your values $\bf c-\cfrac{b^2}{4a}= 12$

jdoe0001 · Answer

and see what you get

jdoe0001 · Answer

keep in mind "a' is negative :/

anonymous · Answer

C does not equal 21 that is not the way to solve the lesson is minimum and maximum 21 is not any of the choices

jdoe0001 · Answer

yes, you'd get 21 if you used "a" as positive, but "a" is negative

jdoe0001 · Answer

you have "$-x^2$" .

anonymous · Answer

No you would get 21 if a is negative do it yourself and you will get that answer you get 3 if it is positive

jdoe0001 · Answer

heheh

anonymous · Answer

Yea that is the wrong way too solve it. This is (Maximum and Minimum

jdoe0001 · Answer

one sec, let's see
$$\bf 
c-\cfrac{b^2}{4a} = 12 \implies 
c-\cfrac{6^2}{4(-1)}=12 \implies c-\cfrac{36}{-4} =12\
 c+9 =12 \implies c = 12 -9
$$

anonymous · Answer

36 divided by -4 is negative 9 not positive 9 sorry

jdoe0001 · Answer

$\bf c-\cfrac{36}{-4} =12\
c-(-9) =12$

jdoe0001 · Answer

$\bf c-\left(\cfrac{36}{-4}\right) =12\
c-(-9) =12$

anonymous · Answer

yes i found the answer and thanks for clarifying but i found it by doing the vertex formula