is this the whole qn?

substitute x=-1 and substitute x=2 into the given equation to get 2 simultaneous equations with 2 unknowns a and b. solve the 2 simultaneous equations to find a and b.

note that value of Q(x) does not matter because when you substitute x =-1 or x=2,
you get 0 x Q(x) = 0!

answer is a = 20 and b = 24.

you can also take a look at Sec 3 Add Maths question uploaded by Silin on 3 Apr (https://ask.manytutors.com/questions/124102).
the question is similar and the method of solving is similar.

@Griff yes this is the whole qns. :)

@Boy Mow Chau
Oooo icic. Tysm for helping out! And also giving me a link to another similar qns so I can practice :)))