Scores are {72, 97, 82, x} where x is the unknown fourth score
They add up to 72+97+82+x = x+251
Divide this sum over four because there are four scores we want to average out. We end up with (x+251)/4 = M where M is the mean. Asa wants the mean to be 82, so make M = 82.
We can replace M with 82 and solve for x like so
(x+251)/4 = M
(x+251)/4 = 82
x+251 = 4*82
x+251 = 328
x = 328-251
x = 77
Asa needs a score of 77 on the fourth test so that the mean of the four tests is 82. If she scores higher than 77 on the fourth test, then the mean will be larger than 82.
-----------------------------------
Depending on what your teacher is looking for, the first step is to establish your variable. So you would say "let x = fourth score". However, that seems too easy. So perhaps your teacher is looking for the equation (x+251)/4 = 82 or some variant of it.