Answer
$\log_{2}$128 = 7
Work Step by Step
The logarithm formula using variables is: $\log_{B}$A = C
Using the same variables, the spots they would go in the exponential formula is: B$^{C}$ = A
So to write $2^{7}$ = 128 in logarithm form, you first have to look at which number corresponds to which variable in the exponential formula. In this case:
2 corresponds to B
7 corresponds to C and
128 corresponds to A
Knowing which number corresponds to which variable in the exponential formula, you can then plug those into the logarithm formula to get your answer which will be:
$\log_{2}$128 = 7