Given,

x² + 15x + 56

Now first find the numbers whose-

Sum = 15 and

Product = 56

Required numbers are 7 and 8,

So we get;

x² + 15x + 56

= x² + 7x + 8x + 56

= x(x + 7) + 8(x + 7)

= (x + 7)(x + 8)