Explanation:
the rules of logarithm.
log(a×b) = log(a) + log(b)
log(a^b) = b×log(a)
2×log(x) = log(16) + log(4)
now, using the above knowledge :
log(x²) = log(16×4)
=>
x² = 16×4 = 64
x = ±8
and since a logarithm of a negative number is undefined the only plausible solution is
x = 8