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The angle θ = 25.9º is an irrelevant distraction. Like F1 and F2, F3 is directed tangentially (perpendicular to the displacement vector) at the point of contact. So you would use a 90º angle.
Torque: τ = r × F
F3 may be directed 25.9º counterclockwise from left but r3 is directed 25.9º counterclockwise from up. The difference in direction is 90º.
τ = ( F1 + F2 )( b ) - ( F3 )( a )
τ = ( 10.7N + 8.55N )( 0.23m ) - ( 12.9N )( 0.7m )
τ = 4.4275 Nm - 9.03 Nm
τ = -4.6025 Nm
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