Instead of doing yours for you, I'll do
one exactly like yours step by step. 
You can then do your equation exactly
the same way:



Divide both sides by 3



Square both sides:



Multiply the exponents on the left,
the outer 2 by the inner exponent 2x+4:



Write 81 as 34



Add the exponents of 3 on the left:



Since the bases are same positive numbers
on both sides and they are not equal to 1,
we may set the exponents equal to each other
and ignore the bases:













Now use the above as a model and solve yours 
exactly the same way, step by step.

Edwin