Because the student scored 73/100 on the first test, their current average is 73%.
To calculate the avergae with the next test score, you add both percentages the student gets together and then divide by 2.
79 = (73 + x) / 2
Multiply by 2 on both sides
158 = 73 + x
Subtract 73 on both sides
x = 85
So the student needs to get at least an 85/100 on the next test to maintain a 79 average.