Your question should be "If an astronaut weighs 98 N on the surface of
moon, then what is his weight on earth’s surface?" as mass of a body remains the same everywhere.
If weight of an object on any planetary body is known, divide it by gravitational acceleration of this planetary body. This gives mass of the body. Now multiply this mass with gravitational acceleration of any other planetary body. This product gives you the weight of the object on the other planetary body.
Thus weight of astronaut on earth will be (98N / g on moon's surface) x g on earth's surface.