The equations of two equal sides `A Ba n dA C` of an isosceles triangle `A B C` are `x+y=5` and `7x-y=3` , respectively. Then the equation of side `B C` if `a r( A B C)=5u n i t^2` is `x-3y+1=0` (b) `x-3y-21=0` `3x+y+2=0` (d) `3x+y-12=0`