Verified answer

r:

y=(5x+1)/4

mr=5/4

s:

y=(kx+2)/3

ms=k/3

mr.ms=-1

5/4.k/3=-1

5k/12=-1

5k=-12

k=-12/5

-5/-4*-k/-3=-1

5k/-12=-1

5k=12

k=12/5