Intro Calculus help

f(x) = x^2+4x-5

Find x such that f(x) = 0

I know it would be:

x^2+4x-5 = 0

(x+5)(x-1)=0

So,the answer would bex=-5orx=1.

But how would I explain that the signs (+ and -) would switch around, as I have to give full working out?

Thanks in advance