Solve for y:
3x = 7y + 21
~Subtract 7y to both sides
3x - 7y = 21
~Subtract 3x to both sides
-7y = 21 - 3x
~Divide -7 to both sides
y = -3 + 3/7x
~Change order
y = 3/7x - 3
Best of Luck!
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