Question 824810
The line h has the slope {{{(10-(-2))/(-4-6)=12/(-10)=-6/5}}}.


The line, k, is also 6y=-ax-12,
{{{y=-(a/6)x-2}}}---------line k.


Line h perpendicular to line k means that {{{highlight_green((-6/5)*(-a/6)=-1)}}}.
The typically stated rule for this is, if two perpendicular lines in a plane have slopes {{{m[1]}}} and {{{m[2]}}}, then {{{m[1]m[2]=-1}}}.