The average of your 3 quiz grades is 17 points. Two of your quiz grades are 14 points and 19 points. Write and solve an equation to find the score of your third quiz.

The equation would be 14+19+x=51, and the 3rd quiz score would be 18.  

Step-by-step explanation:

To get to the average, you have to add all the quiz scores and divide by the number of quizzes. If you work backwards, you can multiply the average by the number of quiz scores, which is 51. Then you subtract the known test scores, and you get your answer.

x = -3

y = -18

step-by-step explanation:

to solve by substitution, substitute one variable and solve for the other. then substitute again to find the remaining variable.

x-1/3(3x-9) = -3 since y=3x-9 substittute y.

x-1/3(3x-9)=-3

x + x +3 = -3

2x+3=-3

2x+3-3= -3-3

2x=-6

x=-3

substitute again into y=3x-9

y=3(-3)-9

y=-9-9

y=-18