A direct proportion is any equation where y = mx+b. (i.e., a linear equation)
Each side of 3y = 1/2x can be divided by 3 to get y = 1/6x, satisfying the condition of y = mx+b. (b=0)
In this case, the constant of proportionality would be 1/6.
Line symmetry this is because if you cut a straight line down the middle it would be the exact same size