The mass density of a metal bar of length 6 m is given by p(x) = 900 + x kilograms per cubic meter. where x is the distance in meters from one end of the bar. What is the average mass density over the length of the entire bar? (Use decimal notation. Give your answer to three decimal places.) tn || How much work is done by a variable force F(x) = (55 - x) N that moves an object along a straight line in the direction of F from x = 4 m to x = 23 m? (Use symbolic notation and fractions where needed.) W =