z^3 + 3z^2 -25z - 75 = 0
Factor out common terms in the first two terms, then in the last two terms
z^2(z+3)−25(z+3)=0
Factor out the common term z+3
(z+3)(z^2−25)=0
Ask: When will (z+3)(z^2−25) equal zero?
When z+3=0 or z^2−25=0
Solve each of the 2 equations above
z=−3,±5
Hope this helps