Question 775957
Perhaps it's easier than I think but:

{{{A = 2(pi)rx + (pi)r^2}}}
{{{A - (pi)r^2 = 2(pi)rx}}}
 
I'm going to rearrange it here so that x is on the left (personal preference).
{{{2(pi)rx = A - (pi)r^2}}}
{{{x = A/(2(pi)r) - (pi)r^2/(2(pi)r)}}}
{{{x = A/(2(pi)r) - r/2}}}