At what distance above the surface of the earth is the acceleration due to the earth's gravity 0.785 m/s2 if the acceleration due to gravity at the surface has magnitude 9.80m/s2?

The acceleration due to gravity falls off as the inverse of the square of the distance from the center of the Earth. Thus the distance of interest is

1 / (d/R) ^2 = 0.785/9.8

d = R·√ (9.8/0.785) ≈ R·3.53328

or R·2.53328 above the surface of the earth.

Wikipedia says the radius of the earth is 6371 km, so your distance is

(6371 km) ·2.53328 ≈ 16,140 km