MMK so saying that a=3 and b=4 means substitute the letters, with the numbers. I'm not gonna do it for you but ill help you out a bit.
5. 2(3)+8=
8. 7(3) x 9(4)=
11. 3^2 + 1=
14. (3)(4)/3=
I think you can do the rest on ur own my friend. Hope this helped :)