How to solve linear equation 3x+y=10

For the equations below which statement is true ?

Atransformation t : (x,y) (x-5,y +3) the image of a(2,-1) is

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?