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## Homework Statement

Let f(x)=a(7-x^2)

find in terms of a, the equations of the lines tangent to these curves at x=-1.

## Homework Equations

?

## The Attempt at a Solution

So I took the derivative of f(x).

f'(x)=a(-2x)+7-x^2

then i plugged in -1 into f'(x)

and i got f'(-1)=2ax+7+1=2ax+8

but the answer to the problem is y=2ax+8a.

I don't know where I went wrong.