6.39
x 0.42
-------------
We can multiply as if it's a normal number and ignore the decimals for now.
6.39
x 0.42
-------------
1278
+25560
--------------
26838
Now, we can count the number of places over the decimal is in each number. In 6.39, there are two numbers after the decimal. In 0.42, there are also two numbers. We add those together to get 4. This is how many places over you will put the decimal in 26838.
Therefore, our answer is 2.6838.