Given,

y² – 21y + 90

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

Sum = - 21 and

Product = 90

The numbers are 15 and 6,

So,

y² – 21y + 90 = y² – 15y – 6y + 90

= y(y – 15) – 6(y – 15)

= (y – 15)(y – 6)