Solve for x: _5(x) = 3.

The equation $\log_5(x)=3$ means that $5$ raised to the power $3$ equals $x$: $x=5^3=125$, choice C. Check: $\log_5125=3$ because $5\cdot5\cdot5=125$. Choice A ($15$) multiplies $5\cdot3$ instead of raising $5$ to the third power. Choice B ($25$) is $5^2$, one power too few. Choice D ($243$) is $3^5$, which swaps the base and the exponent. A logarithm is an exponent: the value of the log is the power, and the base is raised to it.