When determining if you can solve an exponential equation, the following must be true:
The bases on either side of the equation have to be equal.
This allows you to set the exponents equal to each other, making it possible to solve for the variable. If the bases are different, you may need to use logarithms to assist in solving the equation. Additionally, having no variables in the original equation would imply that there is nothing to solve for, which defeats the purpose of solving an equation.