use ac method
for ax^2+bx+c
multiply a and c
find what 2 numbers mutiply to get ac and add to get b
split b into those 2 numbers
group
factor
factor
5x^2+6x-8
ac=5 times -8=-40
what 2 numbers multily to get -40 and add to get 6?
-4 and 10
5x^2+10x-4x-8
group
(5x^2+10x)+(-4x-8)
factor
5x(x+2)-4(x+2)
factor
(5x-4)(x+2)
C is answer