Plzz solve my question

Dear Student
Let the two vectors are A and B with angle θ between them.
 
According to the problem,
A+B=18
and
A2+B2+2AB cosθ=12
Lets say the resultant makes 90 degrees with A.
B2=A2+122=(18-B)2+122B2=324-36B+B2+144B=13
Therefore A=18-13=5 with
θ=cos-1122-A2-B22AB=cos-1144-25-169130=cos-1-513
Magnitude of the forces are 13 and 5.

Regards

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