You need to line up the numbers so the decimal points are aligned. If there are more decimal places for example let’s say 3.14+3.861 you would add them together but put 3.14 as 3.140 so the 1 in 3.861 has a place to go. By the way the answer is 7.001. When subtracting decimals you would do the same thing. For example 6.59-2.418 you would subtract the same way as you would with whole numbers just with decimals. So make 6.59 6.58 with 10 in the thousandths place. A bit confusing but it takes time to understand the concept. So practice! The answer would be 4.172. Hope this helps!