there are three quizzes held
average grades of the three quizzes are calculated by taking the sum of the three quiz grades and dividing by 3
average grade is 17 points
1 quiz grade - 14 points
2nd quiz grade - 19 points
3rd quiz grade - x
the equation we can write to find x is

next we solve it
14 + 19 + x = 51
33 + x = 51
x = 18
therefore third quiz grade is 18