f(1)=0
6(1)^4+a(1)^3-17(1)^2+bx-4=0
a+b-15=0
f(-2)=0
6(-2)^4+a(-2)^3-17(-2)^2+b(-2)-4=0
96-8a-68-2b-4=0
24-8a-2b=0
12-4a-b=0
Now do simultaneous equations
a+b-15=0......1
and
12-4a-b=0.....2
From 2 b=12-4a sub into 1
a+12-4a-15=0
a=-1
Now sub a=-1 into b=12-4a
b=16.
Now...