Given the position of link $4,$ the problem is to determine the postures of links $2$ and $3.$ Given: $OB=30mm,OA=10mm,AB=40mm$ To solve: A $\text{'}$s position. We need which approach below?

F$.C=A+B=B+A.$

$\sqrt[\phantom{2}]{\sqrt[\phantom{2}]{J}}=A+B.$