Given,

x² + 13x + 40

Now first find the numbers whose-

Sum = 13 and

Product = 40

Required numbers are 8 and 5,

So we get;

x² + 13x + 40

= x² + 8x + 5x + 40

= x(x + 8) + 5(x + 8)

= (x + 8)(x + 5)