Seven times the difference of a number and 1

The square root of 9x plus 7 plus the square rot of 2x equall to 7

Two variables, x and y, vary inversely, and x=12 when y=4. what is the value of y when x=3?

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?