The highest common factor is found by multiplying all the factors which appear in both lists
a.(13-4b)(a+6)
13a-24b
13 and 24
so we have to factor this
13 =13×1
24=12×2=12 is 6×2=6 is 3×2=3 is 3×1=3×1
Therefore 1 and 1 get canceled
13×3=39
(a )39
like this we have to solve........