Jackie exerts a 43 N horizontal force as she pulls a sled that weighs 140 N across a cement sidewalk at a constant speed. What is the coefficient of kinetic friction between the sidewalk and the sled?

Answer 1

43 - ckf * 140 = m*a

where ckf is coefficient of kinetic friction. In your textbook ckf is probably represented as the greek letter mu, but it's really hard to write greek letters here. . .
m is the mass of the sled, and a is its acceleration. Since the sled is moving at a constant speed, the acceleration is 0, or

43 - ckf * 140 = 0

Solve for ckf