[clock60]

(y+2-b)/x=m+2/x
y+2-b=mx+2
y=mx+b
for the line to be rotated on 90 degree the slope should be negative and also
inverse i mean that multiply for example k*1/k=1
original slope is m
new one is -1/m
m*(-1/m)=-1
minus for the lines to be perpendicular