I have no clue what the answer is
The ordered pair (-1,5) is a solution to the system because it makes both equations true.
1) x+y=4
2) x-y=-6
1) -1 + 5 = 4
5-1 = 4 you can switch the order to make it easier to understand
so
4 = 4
2) -1 - 5 = -6
-1 + -5 = -6 you can change the sign in the middle and make y -
so
-6 = -6
My Spanish isn't that great but I think you're looking for 18
1) You add 72+89 which you get 161 and you divide by two and you get 80.5
2) you do the same thing 72+89+90=251
And you divide by three and you get ≈83.66
