Find c if the system of equations cx+3y+(3-c)=0; 12x+cy-c=0 has infinitely many solutions?

Given system of equations be cx+3y+(3−c)=0; 12x+cy−c=0
For the system of equations to have infinitely many solutions
a1/a2=b1/b2=c1/c2
c/12=3/c=(3−c)/−c
c/12=3/c
=>c²=36
and c=6
=>c=±6
3/c=(3−c)/−c
=>−3c=3c−c²
=>c²−6c=0
=>c(c−6)=0
=>c=0 or c=6
Since c/12=3/c, c≠0
Therefore c=6