The problem gives you the value of 3x + y.
In the expression, there is 9x + 3 y.
By using the distributive property, we know that 9x + 3y is 3 times greater than 3x + y.
3(3x + y) = 9x + y
Since 3x + y = 21, then we can say:
3(21) = 9x + y
63 = 9x + y
Now that we know the value of 9x + y, we can divide it by the 9 seen in the picture.
63/9 = 7
Therefore, the answer is A.
