This question was popular with most candidates.  However, some of them got it wrong by claiming that tan (P – Q) = tan P – tan Q.  They should understand that tan (P-Q) = tan P – tan Q   which by substituting for tan P and tan Q gives                                                                                               1+ tan P tan Q  
 
tan (p – Q)  =  3–m . When tan (P – Q) = 1, then 3-m    =   1 » m²+m – 2 = 0
                       m²+1                                                m²+1