Since you know you've gone wrong, you're obviously getting an answer different to the given one. It would be useful to see your working, that is, to see how you got your answer. Did you follow this basic approach:

1.

therefore the gradient of the tangent to the curve at P is given by m = .....

2. A point on the tangent to the curve at P is (3, 20).

3. The equation of any line (including a tangent) has the general form

.

4. Conclude that the given answer of

31 = 0 is wrong - that the

31 should be

....