Question

How do you find the product of `-3(4.2)`?

Answer 1

I would write it as `-1*[3xx4.2]` and use the usual "column" multiplication as:
`4.2xx`
`3=`
`126`
Multiplied by `-1` gives `-126`
To place the decimal point you count the decimal places of your factors and move the point from right to left accordingly.
In this case you have only one decimal place (`.2`) so you can place the decimal point before the `6` getting:
`-12.6`

hope it helps