A solenoid with515turns has a length of7.50cm and a cross-sectional area of2.9010⁹m². Find the solenoid's inductance and the average emf around the solenoid if the current changes from +4.00A to4.00A in6.3310³s.

(a)the solenoid's inductance (in H)

(b)the average emf around the solenoid (in V)

For A I got 1.2887 x 10^-6 H... I used the formula L= NA(Mue Knot)N

(515)(2.90 x 10^-9)(4pi x!0^-7)(515/7.5 x10^-2).. Im not sure what i did wrong .

For B I used E = -N(Delta phi beta)/ Delta T. = -L delta i/delta t = (1.2887 x 10^-6)[-4 -(4)/ 6.33 x 10^-3]

but it is also incorrect