Divide both sides by $200$. $(1.05)^x=5$

Take the log of both sides to undo the exponent.
$\log(1.05)^x=\log(5)$
$x\log(1.05)=\log(5)$

Divide both sides by $\log(1.05)$.

1. Add $400$ to both sides.
2. Follow a process similar to the steps in part (a).