Explanation:

a^log_ax = x, here the base of log_ax and number which is based to log_ax are same. i.e. a

Therefore, in the given expression
49^log₇4 = (7²)^log₇4

log_a(x^p) = p(log_ax)

Carefully understand the simplification done using the properties of logarithm.
(7 ²) ^log₇4 = (7) ^(log₇4)2 = (7) ^{log₇(4 2)}

We know that, a^log_ax = x

Hence, (7) ^log₇(42) = 4² = 16