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
Divide both sides by 3
 Square both sides:
Square both sides:
 Multiply the exponents on the left,
the outer 2 by the inner exponent 2x+4:
Multiply the exponents on the left,
the outer 2 by the inner exponent 2x+4:
 Write 81 as 34
Write 81 as 34
 Add the exponents of 3 on the left:
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:
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
Now use the above as a model and solve yours 
exactly the same way, step by step.
Edwin