x^2 + 7x = x (x + 7)
x^2 - 7x = x (x - 7)
lcm is x (x + 7) (x - 7) because x (x + 7) (x - 7) has an x (x + 7) to make x^2 + 7x and it has x (x - 7) to make x^2 - 7x
the lcm is x (x + 7) (x - 7)
or, if you want to expand that out
x (x + 7) (x - 7) = x (x^2 - 49) = x^3 - 49x
the lcm is x^3 - 49x