We first combine the 2 logs on the left into one logarithm.

`log_2\ 7x=log_2\ 21`

`7x=21`

`x=3`

To get the second line, we actually raise `2` to the power of the left side, and `2` to the power of the right side. We don't really "cancel out" the logs, but that is the effect (only if they have the same base).