a² + 6a – 91

Now, first we have to find out the numbers whose-

Sum = 6 and

Product = - 91

The numbers are 13 and 7,

So,

a² + 6a – 91

= a² + 13a – 7a – 91

= a(a + 13) – 7(a + 13)

= (a + 13) (a – 7)