What is the factor expression ok k^2+kf-90f^2

You are trying to factor k^2 + kf - 90f^2.

We start by trying to find two factors of -90f^2 that add up to 1f. The trick is realizing that there is a coefficient of 1 in front of the k here. So, I think the two factors are 10f and -9f. Agree?

Now, you can complete the factors of (k+10f)(k-9f). FOIL it back together to make sure it is correct. Yes!

Good luck.