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**simultaneous polynominals!!**

stuck on this coursework question

The cubic polynominal f(x)= 4x^3+ax^2+bx+6 has a factor (x-2)

when it is divided by (x+1) it has a remainder 0f -15

find the values of a and b

I did as follows

factor (x-2)

f(-2)

4(-2)^3+a(-2)^2+b(-2)+6=0

equals 4a-2b=26

when it is divided by (x+1)

f(-1)

4(-1)^3+a(-1)^2+b(-1)+6=15

equals a-b=-17

when i solve these it gives me b=17 and a=-8.5

Which i know is wrong

plese tell me where

thanks