what is the derivative of y=�ã(3x)

assuming that means √(3x), use the chain rule

d/dx u^n = n*u^(n-1) * du/dx where u=3x and n = 1/2:

y = (3x)^(1/2)
y' = 1/2 * (3x)^(-1/2) * 3
= 3/(2√(3x))