b + 8 x a

Put a 9 where the "a" is and a 4 where the "b" is:

4 + 8 x 9

The PEMDAS rule tells us that we must multiply 8 x 9 before adding to 4.
So since 8 x 9 is 72, we put 72 where the 8 x 9 is:

4 + 72

Finally we add, and the final answer is:

76

Edwin