The formula is x1+x2 divided by 2 and y1+y2 divided by 2.I hope this is what you where looking for
x = 5.
I attached an image of my work!
Step 1: Subtract 8 from both sides
4x = 20
Step 2: Divide each side by 4
x = 5
Well, you could have 2 numbers to get 25 but it is impossible to get 8 unless you divided by a random number