Which is an example of an expression? 7 + d = 56

x - 6 = 23

6n = 34

c + 8

I need an explanation of this and I will mark brainliest

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?