74. Letting B

x

= B

y

= B

1

and B

z

= B

2

and using Eq. 29-2 and Eq. 3-30, we obtain (with SI units

understood)



F

=

q

v

× 

B

4ˆi

− 20ˆj+ 12ˆk = 2

(4B

2

− 6B

1

)ˆi + (6B

1

− 2B

2

)ˆj + (2B

1

− 4B

1

) ˆ

k



.

Equating like components, we find B

1

=

−3 and B

2

=

−4. In summary (with the unit Tesla understood),



B =

−3.0ˆi− 3.0ˆj− 4.0ˆk.