pi is a constant, so there are only 3 variables in this equation. 2 of those are given, so all you need to do is plug in the given values and solve for h:
In 2 2/3 months you ran 76 miles. To know how many miles you ran each month, you can simply use cross multiplication as follows: number of miles per month= (76) / (2 2/3) = 28.5 miles