Solve the quadratic equation x^2 + x - 20 = 0

Factorising the left hand side we get x² + x - 6 = (x + 5)(x - 4). Therefore x² + x - 6 = 0 is the same as (x + 5)(x - 4) = 0. If multiplying two quantities equals 0, this means that one of the two must equal 0. This means (x +5) = 0 or (x - 4) = 0, therefore x + 5 = 0, gives x= -5 or x - 4 =0, gives x = 4 (using the inverse operation).
Therefore, there are two solutions, x= -5 and x=4