Check whether 
is a factor of 
We have to divide 3x3 + 7x by 7 + 3x
If remainder comes out be 0 then 7 + 3x will be a factor of 3x3 + 7x
By long division method,
As remainder is not zero so 7 + 3x is not a factor of 3x3 + 7x
3
Check whether is a factor of
We have to divide 3x3 + 7x by 7 + 3x
If remainder comes out be 0 then 7 + 3x will be a factor of 3x3 + 7x
By long division method,
As remainder is not zero so 7 + 3x is not a factor of 3x3 + 7x