Methods: let BC=x,

Vector AP vector BC= (vector AC+ vector cp). Vector BC

= Vector AC. Vector BC+ vector cp. Vector BC

= 2 times angle ACB+2 times angle (180 degree angle bCB)

= 2 times angle ACB-2 times angle (angle bCB)

= 2x [(4+x2-9)/(4x)-(4+x2-16)/(4x)] Cosine Theorem.

=7/2=3.5