Simultaneous equations
ie let the 2 numbers be a & b
a + b = 3 ... 1
ab = -1 ... 2
From 1, a = 3 - b, let this be eqn 3
Sub 3 into 2
b(3-b) = -1
-b2 + 3b + 1 = 0
Use the quadratic formula, then get a value for b, sub into eqn 1, get a value for a... test them out... and voila, the answer.