Sorry to hear that you already took the test. Here is how to do the problem:
2ˣ = 128
Let's take log base 10 of both sides.
For simplicity, we'll just use the symbol log to mean log₁₀
log (2ˣ) = log (128) Remember that log(yˣ) = x log(y)
x log (2) = log (128) Divide both sides by log(2) to get x by itself
x = log (128) ÷ log (2)