Marginal Cost/Demand and Cost

1. The Marginal Cost of a product is modeled by dC/dx = 4/(x+1)^1/2

When x=15, C=50. Find the Cost function.

2. The demand and cost functions for a product are given by:

p=600 -3x

C=0.3x^2+6x+600

where p is the price per unit, x is the number of units, and C is the total cost. The profit for producing x units is given by:

P=xp - C -xt

where t is the excise tax per unit. Find the maximum profits for excise taxes of t=$5, $10, and $20.

Help for these two questions would be great! thanks