To solve this problem you must follow the proccedure shown below:
1. You have the following function given in the problem:
f(x)=50000(0.8)^x
2. You can rewrite it as below:
y=50000(0.8)^x
3. Then, you must apply interchange the variables:
x=50000(0.8)^y
4. Now, you have:
x/50000=(0.8)^y
5. When you apply ln you obtain:
ln(x/50000)=ln(0.8)^y
ln(x)-ln(50000)=yln(0.8)
6. Now, clear y:
y=[ln(x)-ln(50000)]/ln(0.8)
