Question

KN + 4f = 9v solve for n

Answer 1

there is no n just N 

so for N will be 

N = (9v -4f)/K

hope helped 

Answer 2

this is a literal equation.
first subtract 4f on both sides. kn+4f -4f = 9v -4f
Next divide k on both sides. (kn)/k = (9v - 4f)/k
Simplify to get your answer:
n=(9v - 4f)/k