Please help me with this I have no clue how to do it

Measures of the angle of r, 31 s (x+4) t (3x+9)

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?

If the tip varies directly with the number of guest which equation represents between the tip, t, and the number of guest, g?