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4*y = 4*(7/4)*x-(4*21) // - 4*(7/4)*x-(4*21)
4*y-(4*(7/4)*x)+4*21 = 0
4*y-7*x+4*21 = 0
4*y-7*x+84 = 0 // - 4*y+84
-7*x = -(4*y+84) // : -7
x = (-(4*y+84))/(-7)
x = (4*y+84)/7
x = (4*y+84)/7
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