Simple Algebra Problem

When you have the logarithm as a exponent, and the number that is powered to the logarithm is equal to the base of the log, just cancel everything and the answer is what's left of the log. $=\quad { \boxed { 9 } }^{ \log _{ \boxed { 9 } }{ 4 } }\\ =\quad 4$

Let $log_9 4 = x$

By the definition of a logarithm, if $log_9 4 = x$ , then $9^x = 4$

And that's the answer, the problem is done.