(a)
consider the motion of the tennis ball. lets assume the velocity of the tennis ball going towards the racket as positive and velocity of tennis ball going away from the racket as negative.
m = mass of the tennis ball = 60 g = 0.060 kg
v₀ = initial velocity of the tennis ball before being hit by racket = 20 m/s
v = final velocity of the tennis ball after being hit by racket = - 39 m/s
ΔP = change in momentum of the ball
change in momentum of the ball is given as
ΔP = m (v - v₀)
inserting the above values
ΔP = (0.060) (- 39 - 20)
ΔP = - 3.54 kgm/s
hence , magnitude of change in momentum : 3.54 kgm/s
Answer:
Part a)

Part b)

Explanation:
Part a)
As we know that the magnetic force on the current carrying wire is given as

so we have


so we have

Part b)
Now magnetic field is changed to 0.55 T
so we will have


Im pretty sure it’s a because it makes more sense you know?.

Actually Welcome to the concept of resistors,
The equivalent circuit is given as,
1/Rp = 1/3 + 1/6 + 1/9
1/Rp = 6+3+2/18
1/Rp = 11/18
Rp = 18/11 ==> 1.63 ==> 1.6 ohms
hence the equivalent resistance of the circuit is 1.6 ohms