How do I find the drag force and the magnitude of the drag force

A baseball with mass .145 kg is pitched at 39.0 m/s. Upon reaching home plate, 18.4 m away, its speed is 36.2 m/s. If the decrease is due entirely to drag, find (a) the word done by the drag force and (b) the magnitude of the (assumed constant) drag force.

Answer (a) -15.3 J; (b) 0.830 N

How do you get these answers?

a)
cons. of energy
1/2m(vi^2-vf^2)+W=0
then
W=-0.5*0.145(39^-36.2^2)
W=-15.26 J ~ -15.3 J

b)
W=Fx
F=W/x
F=15.3/18.4=0.83 N

ΔK=W
1/2m(vf^2-vi^2)=W
then
1/2m(vi^2-vf^2)=-W
and
1/2m(vi^2-vf^2)+W=0

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