Given:
#(2x)/3-3/4=x/6+21/4#
All denominators are factors of #12#. Multiply both sides by #12#.
#color(red)cancel(color(black)(12))^4((2x)/color(red)cancel(color(black)(3))^1)-color(red)cancel(color(black)(12))^3(3/color(red)cancel(color(black)(4))^1)=color(red)cancel(color(black)(12))^2(x/color(red)cancel(color(black)(6))^1)+color(red)cancel(color(black)(12))^3(21/color(red)cancel(color(black)(4))^1)#
Simplify.
#4xx2x-9=2x+63#
#8x-9=2x+63#
Add #9# to both sides of the equation.
#8x=2x+63+9#
Simplify.
#8x=2x+72#
Subtract #2x# from both sides.
#8x-2x=72#
Simplify.
#6x=72#
Divide both sides by #6#.
#6x=72/6#
Simplify.
#x=12#