Determine its position u for equilibrium 

Okay so if you were having the same issue as me here is the reason: cos(a+b)=cosa*cosb-sina*sinb cos(a+b)+sina*sinb=cosa*cosb sina*sinb-cosa*cosb=-cos(a+b) in the video, he said that it equalled pos cos(a+b), but it actually equalled neg cos(a+), which then makes his answer correct! Hope that helps anyone who else who got confused there!