First, subtract #color(red)(2pir^2)# from each side of the equation to isolate the #h# term:
#S - color(red)(2pir^2) = 2pirh + 2pir^2 - color(red)(2pir^2)#
#S - 2pir^2 = 2pirh + 0#
#S - 2pir^2 = 2pirh#
Now, divide each side of the equation by #color(red)(2pir)# to solve for #h#:
#(S - 2pir^2)/color(red)(2pir) = (2pirh)/color(red)(2pir)#
#(S - 2pir^2)/color(red)(2pir) = (color(red)(cancel(color(black)(2pir)))h)/cancel(color(red)(2pir))#
#(S - 2pir^2)/color(red)(2pir) = h#
#h = (S - 2pir^2)/color(red)(2pir)#
Or
#h = S/color(red)(2pir) - (2pir^2)/color(red)(2pir)#
#h = S/(2pir) - (color(red)(cancel(color(black)(2pi)))r^2)/cancel(color(red)(2pir))#
#h = S/(2pir) - r^2/r#
#h = S/(2pir) - r#