At what altitude above the Earth's surface is the acceleration due to gravity equal to g/7

I tried using r^2 = (G*Me)/a
r^2 = [(6.673e-11)(5.97e24)]/(9.81/7)
But when I get r, I don't know what to do with it, help please.

This is a classic example of why you shouldn't just memorise formulae but make the effort to learn the physics behind the equations.

Remember Newton's two formulae F = (G m1 m2)/r^2 where G is Newton's gravitational constant, m1 is the mass of the earth, m2 is the mass of the falling object and r is the distance from the centre of the earth to the centre of the mass of the object. Obviously this is a general formula and works for any two masses not just the earth but for this example we're using the mass of the earth and the falling object.

We want acceleration though not just force but we know F = ma (where m is the mass of the falling object m2 in our previous equation). So F/m = a, but we know F = (G m