y-y1=m(x-x1) where (x1, y1)=(1, -7)
y+7=-4(x-1)
y+7=-4x+4
y+7=(-4x+4)*4
y+7=-x+16
y=-x+9
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y=9−x
y−9=−x
−y+9=x
9−y=x
x=9−y
y*x=9 put 1 in for x
y=9
