5.9 × 10^24 = 5,900,000,000,000,000,000,000,000 minus 13,000,000,000,000,000,000,000 equals 5,887,000,000,000,000,000,000,000 kg which is the amount greater that Earth's mass is. you could write this problem as 5.9 × 10^24 - 13 × 10^21 = 5.887 × 10^21
