Given,

q² – 10q + 21

Now first find the numbers whose-

Sum = - 10 and

Product = 21

Required numbers are 7 and 3,

So we get;

q² – 10q + 21

= q² – 7q – 3q + 21

= q(q – 7) – 3(q – 7)

= (q – 7)(q – 3)