Question 902044
Using knowledge of standard form equation for a line, ax+by=k will have slope {{{-a/b}}}.


cy+4x=9 has slope {{{-4/c}}}.


cy-5x=10 has slope {{{5/c}}}.


The two lines given to be perpendicular requires that the product of the slopes equals negative one, {{{highlight_green(-(4/c)*(5/c)=-1)}}}.  Solve for c.


{{{-20/c^2=-1}}}
{{{20/c^2=1}}}
{{{20=c^2}}}
{{{c=0+-sqrt(4*5)}}}
{{{highlight(c=0+- 2*sqrt(5))}}}.