The formula for averaging things is (a+b+c+d)/4 (because there are 4 items being added.)
So we want the average to be 80, so we set things equal to 80.
Let the score for the unknown test score be x.
80 = (84+78+82+79+x)/5
400 = 84+78+82+79+x
x = 77
The lowest possible score Alexandria can get is 77 points.