Question

Solve x2 + 15x = –56.

Answer 1

15x = 7x + 8x

$x^2+15x=-56\\\\ x^2+15x+56=0\\\\ x^2+7x+8x+56=0\\\\x(x+7)+8(x+7)=0\\\\ (x+7)(x+8)=0 \\\\ x=-7 \ or \ x=-8$

Answer 2

An alternative answer to the correct one by Mjankiewicz is this:

You move -56 to the left of equal with changed sign

x² + 15x + 56 = 0

The formula to solve this is:

x1,2 = (-b±√Δ)/2a

Δ = b²-4ac

15 is b, 56 is c and 1(x) is a

x1,2 = (-15±√15²-4*1*56)/2*1

x1,2 = (-15±√225-224)/2

x1,2 = (-15±√1)/2

x1,2 = (-15±1)/2

x1 = (-15+1)/2 = -14/2 = -7

x2 = (-15-1)/2 = -16/2 = -8