The formula is
A=pe^rt
A future value 250
P present value 100
E constant
R interest rate 0.09
T time?
250=100e^0.09t
Solve for t
250/100=e^0.09t
Take the log
Log (250/100)=log (e)×0.09t
0.09t=log (250/100)÷log (e)
T=(log(250÷100)÷log(e))÷0.09
T=10.18 years round your answer to get 10 years
Hope it helps!