#### Answer

(a) The force exerted by the air is 20 N
(b) The cyclist's speed will be $6.67~m/s$

#### Work Step by Step

(a) Since the cyclist is moving at a constant speed, the force exerted by the air must be equal in magnitude to the force pushing the cyclist forward. We can find the magnitude of the force:
$P = F~v$
$F = \frac{P}{v}$
$F = \frac{120~W}{6.0~m/s}$
$F = 20~N$
The force exerted by the air is 20 N
(b) We can find the cyclist's speed when the force exerted by the air is 18 N:
$F~v = P$
$v = \frac{P}{F}$
$v = \frac{120~W}{18~N}$
$v = 6.67~m/s$
The cyclist's speed will be $6.67~m/s$