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
