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:
Now take the limit
When a = 3 the value this limit is equal to .
This means that the gradient of the tangent line is .