if cos x= 3cos y then 2tan (y-x)/2 is equal to

i want answer urgently

There are infinite solutions between 0 and infinite. Note two solutions at extremes (1) when x=90^o and y=90^o and (2) when x=90^o and y=270^o. There are several solutions that satisfies cos(x)=3cos(y) and hence the 2tan ((y-x)/2). It looks like your question is incomplete or there are other constraints like x and y are within a specified domain.