How do you solve this math questions

f(x)= x^3+ax^2+bx+8
when f(x) is divided by (x+1), the remainder is 18
when f(x) is divided by (x-2), the remainder is 0
Find the values of a and b

This question has really got me stumped, Any help?

f(-1) = 18 = -1+a-b+8
f(2) = 0 = 8+4a+2b+8
a-b = 11
2a+b = -8
3a = 3
a = 1
b = -10

use synthetic division, then you get two equations with 2 unknowns
-1 1...a...b...8