Factor the expression.
k^2 + 5kf - 50f^2

a. (k-10f)(k+5f)
b. (k+10f)(k-5f)
c. (k+10f)(k+5f)
d. (k-10f)(k-5f)

B. (k+10f)(k-5f) you can tell in 2 ways.  1. if the second sign is neg then the signs in the factors are going to be one neg and one positive so that knocks out C and D.  Then you look at the second term and since the 5 is positive then the 10 has to be positive so you end up with a positive 5kf.

The second way is to FOIL them out.  When you check B you get  combine your like terms and you have your original expression of 

Hope that helps