What must be added to 3x-7 to make x^24x-1

You need to add: x2 + x+6

Explanation:

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