The start is that you have the form y = 125 (a)^x for an exponential function.
Since 125 (aka 2008) is your starting year, you can substitute in x=4 and y=145, since 2012 is 4 years after 2008. (2012-2008=4)
145 = 125 a^4
Divide by 125 on both sides:
145/125 = a^4
Take the 4th root of both sides on a calculator and you’ll have your a-value.