What constant would be added to the expression x2−7xx2−7x in order to make it a perfect square?

Ok

assuming ya mean
since the quadratic coefient is 1, take 1/2 of the liner coeficient and square it
the linear coefient is -7
half of that is -7/2
squared we get 49/4
add that to get

it factors to
or

you must add to make it a perfect square