You want to solve: 

3^(x+1)-3^x=72

If you substitute with 't = 3^x' the equation becomes:

3*t-t=72 (Since 3^(x+1) = 3*3^x)

Solving the above equation for 't' gives the following solution:

t = 36

Now, lets go back to 'x':

3^x = 36

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Click here to see the solution for 'x'

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