Let A be Arsenal's score, E be Everton's score, and L be Liverpool's score
Each piece of information gives us an equation:
"Arsenal and Everton together: 91" gives A+ E= 91
"Arsenal and Liverpool together: 82" gives A+ L= 82
"The three teams together: 117" gives A+ E+ L= 117
So you have three equation so solve for the three values, A, E, and L. You want to reduce from three equations in three unknowns to one equation in one unknown.
There are many different ways to do that. For this problem, I notice that the first two equations involve only two of the unknowns, each, and "A" is in both. So solve the first equation for E in terms of A; subtract A from both sides to get E= 91- A. Solve the second equation for L in terms of A; L=82- A.
Now replace E and L in the third equation by those:
A+ E+ L= A+(91- A)+ (82- A)= (A- A- A)+ (91+ 82)= -A+ 173= 117.
Can you solve that equation for A? Once you have A, of course, E= 91- A and L= 82- A.