Question

What two numbers multiple to get 4 and add to get -5?

Answer 1

a*b = 4
a+b = -5
a = -5-b

Substitute the value of a in first equation, 
(-5-b)*b = 4
-b²-5b-4 = 0
-b²-b-4b-4 = 0
b(-b-1) 4(-b-1)= 0
b+4 = 0  OR  -b-1 = 0
b = -4  OR b = -1

Substitute it in first equation,
a-4 = -5    OR   a-1 = -5
a = -5+4   OR   a = -5+1
a = -1       OR   a = -4

So, a = -1 & b = -4 or vice versa

Hope it helped.