Is that 5^x=81.2?
5^x=81.2 take the natural log of both sides
xln5=ln81.2 divide both sides by ln5
x=ln81.2/ln5
x≈2.7320 (to nearest ten-thousandth)
If you just meant 5^x=81, the process is the same
5^x=81 take the natural log of both sides
xln5=ln81 divide both sides by ln5
x=ln81/ln5
x≈2.7304 (to nearest ten-thousandth)