c=-2 so overall y=2x-2?
The problem I think is that you don't understand the meaning of the derivative enough to understand the exercice. So, you have given a function and you have to find the gradient at the point which you have found: 2 and this is the gradient of the tangent line at the point , the only thing you need now is the point which is (1,0) which lies on the tangent line.
So this means you just have to substitute these values in the equation of a straight line:
where m=gradient, and a point on the line.