3 · -8 + 5 - 4 ÷ 2= please help will mark as brainiest

X+ y = 5 ; x - y =3 solve using linear combination

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?

50 points~ ! i have to turn this in tomarrow