I don't need help on this one

The table above shows approximate conversions from gallons to liters. what is the value of x?

Ineed to know the answer to this algebra question.

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?