Question

What must be added to 3x-7 to make x^2+4x-1

Answer 1

3x-7+c=x^2+4x-1
find c
minus 3x from both sides
-7+c=x^2+x-1
add 7 to both sides
c=x^2+x+6

x^2+x+6 must be added

Answer 2

The term that can be added is the difference between the two expressions you have:

x^2+4x-1-(3x-7)

x^2+4x-1-3x+7

x^2+x+6