If you are looking at GCSE questions then you have to estimate it as the problem with curves is that the gradient keeps changing.
You draw a line that just touches the curve at the point you need. Make sure it doesn't CROSS the curve, just touches it.
Then you work out the gradient of the line as usual!
If you are doing A Level (or IGCSE maths or GCSE Further Maths) then you need to use differentiation. Have a look at this to explain further.