I'll start at the start. First of all, you should make life as easy for yourself as possible.

So write as .

I'll do it for general a. If you prefer, you can replace all the a's with 3 .......

Note that if f(x) = 1, then since f(x) = 1 for all values of x including x = a + h and x = a.

So you only have to worry about getting for :

Put over a common denominator:

Simplify:

.

So .

Now take the limit

.

When a = 3 the value this limit is equal to .

This means that the gradient of the tangent line is .