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y-50 = (-21/4)*(x-10) // - (-21/4)*(x-10)
y-((-21/4)*(x-10))-50 = 0
(21/4)*(x-10)+y-50 = 0
21/4*(x-10)+y-50 = 0
21/4*(x-10)+y-50 = 0
21/4*x+y-105/2-50 = 0
21/4*x+y-205/2 = 0
21/4*x+y-205/2 = 0
21/4*x+y-205/2 = 0 // - y-205/2
21/4*x = -(y-205/2) // : 21/4
x = (-(y-205/2))/21/4
x = 4/21*(205/2-y)
x = 4/21*(205/2-y)
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