can anyone do my math with me, for free? it's algebra 1 and im in 8th grade, it'd mean a ton !! i desperately need my grade to be a b and it's a f. (59% need to get to 80%)

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?