Find dy/dx for y = sq. root (2x+1) * x^3

Use the rules for the derivative of a product, and the function of a function.

Here is a start: d/dx(2x + 1)^1/2
= (1/2)(2x +1)-1/2 * 2
= 1/sqrt(2x+1)

You should be able to apply the product rule:
d/dx (u*v) = u dv/dx + v du/dx
where u and v are functions of x.