I’m confused how do you know where to put the plus and minus signs when rearranging the equation?

Math it's appreciated. (picture provided)

On a cm grid, point p has coordinates (3,-1) and point q has coordinates (-5,6) calculate the shortest distance between p and q give your answer to 1 decimal place

Which angle is the smallest? also give me an explanation so i can complete it to other questions as well, : )