ok well we know that the slope is -2 so that gets 1 out of the options, now you want to find the y value to get your last number well I will highly recommend you graph it so you know how to do it for a next time, you graph it by plugging the points they gave you adn using your slope figure it out, you plug in (3,1) and then rise 2 and run 1 toward the left side, because your slope is negative this gives you a y-intercept of 7 so your answer would be y=-2x+7,
hope this helps