F_s= μ_s × N
To get newtons is kind of convoluted but I believe the method is: mass × gravitational pull = Newtons.

Front Seat:

F_s= μ_s × N
Newtons = (Black’s weight + books) × 9.81; Newtons= 490.5
F_s= .5_s × 490.5 = 245.25

Back Seat:

F_s= μ_s × N
Newtons = Black’s weight) × 9.81; Newtons= 392.4
F_s= .7_s × 392.4 = 274.68

Answer

So, she should sit in the backseat.

Now. What is the what is the coefficient of static friction for the trunk?