21a2-a-10

Factor -1 out of -a.

21a2-(a)-10

Rewrite -1 as 14 plus -15

21a2+(14-15)a-10

Apply the distributive property.

21a2+14a-15a-10

21a2+14a-15a-10

Group the first two terms and the last two terms.

(21a2+14a)-15a-10

Factor out the greatest common factor (GCF) from each group.

7a(3a+2)-5(3a+2)

7a(3a+2)-5(3a+2)

Factor the polynomial by factoring out the greatest common factor, 3a+2.

(3a+2)(7a-5)

Factor 21a^2-a-10