Cartesian coordinates and torque

[RhysticGenesis]

This section I don't understand at all... but the problem is What is the torque tau_B due to force F_vec about the point B? (B is the point at Cartesian coordinates (0, b), located a distance b from the origin along the y axis.)
Express the torque about point B in terms of F, theta, phi, pi, and/or other given coordinate data. an image can be found at
http://session.masteringphysics.com/problemAsset/1011042/19/MRB_rk_0.jpg

I put in F*b*sin(pi-(theta)) it noted that I was wrong... I don't understand? and that's just hte beginning of my problems in this section

 

[HallsofIvy]

The torque "due to force F around point B" is the product the distance from B to the point at which F is applied and the component of F perpendicular to the line from B to the point where F is applied.

 

[RhysticGenesis]

so b*F*tan(pi-(theta)) ? I still don't understand I know if I do sin(pi-(theta)) Its opp over hyp so it would be parallel to b but I am not sure what trig woudl get it to go perp then