Magnitude of magnetic field at distance from central axis

[qlzlahs]

Homework Statement

A coaxial cable consists of an inner cylindrical conductor of radius R1 = 0.040 m on the axis of an outer hollow cylindrical conductor of inner radius R2 = 0.080 m and outer radius R3 = 0.090 m. The inner conductor carries current I1 = 4.4 A in one direction, and the outer conductor carries current I2 = 6.9 A in the opposite direction. What is the magnitude of the magnetic field at the following distances from the central axis of the cable?
(μ0 = 4π × 10-7 T · m/A)
(A) At r = 0.060 m (in the gap midway between the two conductors)
(B) At r = 0.150 m (outside the cable)

Homework Equations

1. B = (μ_0/(4pi))*(Q)*(v x r)/(r^2)
2. B = ((μ_0/(2pi))*(I)*(r^2 / R^3)

The Attempt at a Solution

I'm just really not sure how to solve this. Do I plug this in the 1st equation or the second? What is r (and R)? Is there a different equation I can use?

 

[Orodruin]

Do I plug this in the 1st equation or the second?

This right here is your problem. Physics is not about finding a formula to plug things in to. In order to learn you need to understand in which situations your formulae are applicable. So what do the formulae you quoted eescribe?

 

[qlzlahs]

In order to learn you need to understand in which situations your formulae are applicable

Whoops. So I think I use this equation for magnetic fields in cylindrical conductors?:
B = (µ_0*I*r) / (2π*a^2), where a is the radius and r is the distance.