Using the information you have and the formula for the sum to n terms of an aritmetic sequence construct 2 equations with the variables a and d as follows:

First using sum of first 2 numbers is 18:

Then the second piece of information sum to 4 is 52:

All you need to do now is solve simultaneously to find a and d. Then using these values use the same formula again to find the sum to 8 terms.