$4-$ For the double$-$slider linkage in the posture shown, the angular velocity of the input crank $2$ is $42$ rad$/$s clockwise. Find the velocities of points B$,$ C$,$ and D using either $($a$)$ Velocity difference graphical method or $($b$)$ Instant center graphical method. RAO$2=2$in$,$ RBA$=10$in$,$ RCA$=4$in$,$ RCB$=7$in$,$ RDC$=8$in