Hi Brittany
A = A0e-0.01155x
We begin with 900 pound and want to know the time that will pass for only 357 pound to remain
357 = 900e-0.01155x
We need to isolate the x (time)
Divide by 900
.39666... = e-0.01155x
Take the natural log of both sides
ln (0.39666...) = -0.01155x
-0.92466 = -0.01155x
And finally divide by -0.01155 to solve for x
x = 80 years