Explanation:
log2(20) - log2(x) = log2(20/x)
log2(5) + log2(x) = log2(5x)
so, we have
log2(20/x) = log2(5x)
20/x = 5x
20 = 5x²
x² = 20/5 = 4
x = ±2
x = 2 makes all arguments for the log funding positive, and is therefore a valid solution.
x = -2 makes the arguments of some log functions negative (e.g. log2(x)). this is impossible, so, x = -2 is an extraneous solution.