First of all, you need to know these two equations:
a^b = c, which also translates into: log a c = b
Now, let us apply these equations here:
a here will stand for 2, b will stand for x, and c will stand for y.
So:
2^x = c, which translates into log2y = x
This means that the equation which is equivalent to y=2x is B. x = log2y
