The answer you are looking for could either be 16 or 40. To solve the equation, you would follow the steps of PEMDAS. Since the 2 above the equation is an exponent, you'd first solve there.
Fill in "a" and "b", the equation will now say 2^2 * 3 + 4 = ?. Assuming that the exponent is meant to go with the 2 alone, 2 * 2 = 4. This leaves the equation to say 4 * 3 + 4 = ? Multiply 3 and 4 to get 12, then add 4 to get 16.
OR
Fill in "a" and "b". This time, we're assuming that the exponent is going with 2 * 3 (originally 2a). Multiply 2 and 3 to get 6, then square 6 to get 36. Finally, add 4 to 36 to get 40.
I'm not quite sure where the exponent was meant to go, but I hope this helps!