The equations of the sides \( AB \), \( BC \) and \( CA \) of a triangle \( ABC \) are \( 2x+y=0 \), \( x+py=15a \) and \( x-y=3 \) respectively. If its orthocentre is \( (2,a) \), \( -\dfrac{1}{2}<a<2\), then \(p\) is equal to________.
