Find the answer m<i

Evaluate the expression for the given value of the variable 2×(c2-5) for c=4

The perimeter of a rectangle is 42 inches. if the width of the rectangle is 6 inches, what is the length

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?