Question 1013745
The two equations have only one point in common and it is given, (4,6).  The other equation starts as possible {{{3x+By=C}}}, and using the known point, 
{{{3*4+6*B=C}}}
{{{6B+12=C}}}


You can pick any real number you want for B and evaluate the corresponding C.  You have infinite solutions for this.  As example, if B=-1, then C=6, and one possible equation is  3x-y=6.  There are infinitely many possible equations you can choose this way.