The formula for the Midpoint is (xm, ym)=((x1+x2)/2 , (y1+y2)/2 ) where the subscript m stands for midpoint and (x1,y1) and (x2,y2) are the two endpoints. In this problem we are given one of the endpoints (-5,1) as well as the midpoint (-3,3).
The easiest way to solve this is to split the equation up so that we solve for x by itself and then y by itself.
So we get xm=(x1+x2)/2. We plug in the x coordinate of the midpoint to get -3=(x1+x2)/2. Then we plug in the x coordinate of one of our endpoints to get -3=(-5+x2)/2. If we multiply both sides by 2 we get -6=-5+x2. Then we add five to both sides to get -1=x2. So the x coordinate of our other endpoint is -1.
Now we repeat a similar process to solve for y. Our y coordinate of the midpoint is 3. So we have 3=(y1+y2)/2. Then we plug in the y coordinate for our original endpoint to get 3=(1+y2)/2. We multiply both sides by 2 to get 6=1+y2. Subtract 1 from both sides to get 5=y2. So the y coordinate of our other endpoint is 5.
We put these two pieces of info together to show that our other endpoint is (-1,5).
The faster way to solve this problem is to think about it logically and this can be used to check your answer. If we started at an x coordinate of -5 and we know that at the midway point the x coordinate was -3 then that means we moved up +2 on the x axis. To go the other half of the distance we would add +2 again. This would take us from -3 to -1 on the x coordinate. This lines up with the way we solved it numerically.
Repeating the same process for y, we started at 1 and added +2 to get to our midpoint of 3. If we add two more that will take us to the other side of the midpoint and we will end up with 3+2=5. This also checks with how we solved the problem numerically