Help me out please. This is Algebra 2 problem.

How many grams must be adde to 970g to make 1 and a half kg

Ihave trouble finding the nth term of a linear sequence

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?