Aforce in the +x-direction with magnitude f(x) 18.0 n- (0.530 n/m)ax is applied to a 7.80-kg box that is sitting on the horizontal, frictionless surface of a frozen lake. f(x) is the only horizontal force on the box if the box is initially at rest at = 0, what is its speed after it has traveled 15.0 m? - express your answer to three significant figures and include the appropriate units.