third question I'm giving 50 points

Answer: B
Step-by-step explanation:
y = mx + mc
The perpendicular will have slope -1/m and intercept mc, which is
y = (-1/m)x + mc
For the answers you have to factor out a -1/m from both terms:
[tex]$y = -\frac 1m x + mc \cdot -m \cdot -\frac 1m$\\[/tex]
[tex]$y = -\frac 1m (x + mc \cdot -m)$[/tex]
[tex]$\boxed{\;\textbf{(B)}\ \,y = -\frac 1m (x - m^2 c)\;}$[/tex]