Find the value of k for which the system of equations 6x + 7y = 0, kx + 21y = 0 has infinitely many solutions.
The given system is of the form a1 x + b1 y = 0, a2 x + b2 y = 0
a1 = 6, a2 = k, b1 = 7,b2 = 21
If the system has infinitely many solutions.
a1a2 = b1b2
6k = 721
k =18