5a (2x-a) - (8a-x) (2x-a)

Remember foil and distributive property

note the invible - 1

5a (2x-1) - 1 (8a-x) (2x-a)

look at first bit

5a (2x-a) = 10ax-5a^2

2nd part

-1 (8a-x) (2x-a)

-1 (16ax-2x^2-8a^2+ax)

-1 (17ax-2x^2-8a^2)

-17ax+2x^2+8a^2

now combine the 2 parts

10ax-5a^2-17ax+2x^2+8a^2

2x^2+3a^2-7ax