Question

if (x+5) is a factor of p(x) = x3-20x+5k, then k = ?

Answer 1

put x=-5 in p(x)
-5*3-20(-5)+5k=0
5k=-85
k=17

Answer 2

When x+5 is a factor of p(x) then x=5 will satisfy the polynomial so
P(5)=0
5*3-20*5+5k=0
15-100+5k=0
-85+5k=0
5k=85
K=17