Choose the value of the x determinant (Dx) in the following system. Can you please show me step by step? I'm really confused and I still haven't been able to find the answer.

3x – 4y = –7
x + y = 7

Question

Choose the value of the x determinant (Dx) in the following system. Can you please show me step by step? I'm really confused and I still haven't been able to find the answer.

3x – 4y = –7 
x + y = 7

zpupster · Answer

find the denominator first: take the coefficients of x and y

|3  -4|
|1    1|

cross multiply 3 x 1  and 1 x -4 = 3 -(-4)    = 3+4 = 7

to get the numerator
next substitute the x coefficients with the constants on right had side of equations

|-7  -4|
| 7     1|

cross multiply like before
-7 - (-28) = 21

then 21/7 = 3  this our actual x

lets solve for y

x+y = 7
3 + 4 = 7

and in the other equation
3(3) -4(-4) = -7
9-16 = -7

anonymous · Answer

@zpupster thank you for responding! That's what I got too, but I'm still confused because the only choices I have to choose from are: 21, 28, -14 or -35. ???

zpupster · Answer

21 is you DX

zpupster · Answer

we got carried away and solved, it is just asking for DX

anonymous · Answer

@zpupster you're my hero! Thanks so much :~)