The mass of Jupiter is 1.898 * 10 (27) kilograms, and the mass of Saturn is 5.68 * 10 (26) kilograms. How much greater is Jupiter's mass than Saturn's mass?

Just subtract but take care of the power or else you can come up with wrong results!

so first make the power same;

for Jupiter = 1.898 * 10 (27) = 18.98 * 10 (26)

for Saturn = 5.68*10 (26)

so now subtract = (18.98-5.68) * 10 (26)

= 13.3 * 10 (26)

so the answer is 13.3 * 10 (26)

or, 1.33 * 10 (27)