The first equation is
y = 6x - b
For the system to have an infinite number of solutions, both equations must be the same equation.
Solve the second equation for y.
-3x + 1/2 y = -3
1/2 y = 3x - 3
y = 6x - 6
Compare this with the first equation,
y = 6x - b
That means b = 6
