What is a linear function in the form y=mx+b for the line passing through (4.5, -4.25) with y-intercept 2.5?

Contrast the force of gravity between these pairs of objects: 1-kg mass and a 2-kg mass that are 1 m apart; a 1-kg mass and a 2-kg mass that are 2 m apart; and two 2=kg masses that are 1 m apart. i dont understand.