To find a, you need to isolate/get the variable "a" by itself in the equation:
[tex]\frac{1}{2} (a-7)=33[/tex] Multiply the inverse of 1/2, which is 2, to get rid of the fraction
[tex](2)\frac{1}{2}(a-7)=(2)33[/tex]
a - 7 = 66 Add 7 on both sides to get "a" by itself
a - 7 + 7 = 66 + 7
a = 73
