The three scores are 86, 74, and an unknown score x. The average (i.e. the sum divided by how many there are) is 80, so we have
Multiply both sides by 3 and you get
Subtract 160 from both sides to get
Note: you could solve this exercise very quickly if you notice that the two numbers, 86 and 74, are symmetrical with respect to 80, which is the average. So, the third number must be the average.