Can someone help my factor: 5x^2-21=-4 (I can't seem to get the right numbers..)

Question

hartnn · Answer

5x^2-21+4=0
so tell me 2 numbers whose product is 20 and sum is -21

anonymous · Answer

I want to say 7 x -3 ? but that doesn't make sense...or at least when I tried doing it, it did not fit in correctly...

hartnn · Answer

yes, 7,-3 are not correct,
the two numbers are simple to find....try again

anonymous · Answer

I don't know ..

hartnn · Answer

try -20 and -1

anonymous · Answer

WOW....I feel dumb. Thanks for helping me out.

hartnn · Answer

welcome....

hartnn · Answer

so what factors u got ?

anonymous · Answer

I plugged in the -21 and -1 : (5x-21)(x-1) and I got 5x^2-5x-21x+21 ..that's wrong isn't it..

hartnn · Answer

5x^2-21x+4=0
yeah, coz u need to split the middle term 
5x^2-20x-x+4=0

anonymous · Answer

What? I didn't learn how to split terms...I still don't get how you got that...
and how would you factor it?

anonymous · Answer

wait would the equation become 5x(x-4)(x+4) since you factor out the GCF?

hartnn · Answer

5x^2-20x-x+4=0
5x(x-4)-1(x-4)=0
now factor out x-4

anonymous · Answer

2 and -2 ?

hartnn · Answer

5x(x-4)-1(x-4)=0
(x-4)(5x-1)=0
and u are done factoring.....