You need to add: x2+x+6
Let the unknown part be represented by A
Then we have: 3x−7+A=x2+4x−1
Subtract 3x from both sides
−7+A=x2+4x−3x−1
but 4x−3x is giving:
−7+A=x2+x−1
Add 7 to both sides
A=x2+x+6
So you need to add: x2+x+6
