whenever we solve an equation we want to get the unknown (x) alone on one side of the equals sign, but remember whatever we do to one sign of the equation, we have to do to the other. So lets start by subtracting 11 from both sides:
x/2 + 11 = 15
x/2 = 15 - 11
this then works out to
x/2 = 4
from here we can multiply both sides of the equation by 2:
x = 4 x 2
therefore x = 8