I just did this question as well
The best way to answer this is by using the distance formula.
so lets say sqrt(x2-x1)^2 +(y2-y1)^2 = sqrt(x2-x1)^2+(y2-y1)^2
I would start by graphing those points and y=x and then use the distane formula to solve the question. Refer to this site for more help with it! The Distance Formula