Y2 + 15y +56 = (y + 7)(y+)
y +7
y - 7
y-8
y - 8

Find the number after y+

y^2 + 15y + 56 factors to (y+7)(y+8)

This is because 7+8 = 15 is the middle term and 7*8 = 56 is the last term. We can use the box method, FOIL rule, or distributive property to confirm we have the right factorization. I'll use the distributive rule

(y+7)(y+8) = y(y+8) + 7(y+8)

(y+7)(y+8) = y^2 + 8y + 7y + 56

(y+7)(y+8) = y^2 + 15y + 56