Whenever a numerical value is given in terms of some units, the slogan to remember is:

•     the units are part of the number

This applies to any numerical value anywhere in an equation, unless that number is a dimensionless coefficient.  If the numerical value is part of a larger expression, a good practice is to group the number and its units in parentheses, to avoid any ambiguity. Some correct examples:

F_x  = m * a_x
m = 10 kg
F_x = (10 kg) * a_x
vf_x^2 = vi_x^2 + 2 * a_x * d_x
vf_x^2 = (10 m/s)^2 + 2 * (3 m/s^2) * (5 m)

• You should not include units when using a variable. You only include units with numerical values. As you see, there are no units written next to variables like F_x and a_x, but units are included as part of numerical values like 10 kg.

Remember that when you multiply values, you multiply their units as well. That is, the units of the whole product are the product of the units of the parts:

F_x = (10 kg) * (5 m/s^2)
F_x  = 50 kg*m/s^2
F_x = 50 N           ; equivalent to previous equation, since 1N = 1 kg*m/s^2

In some cases you may want to express a value as the product of a dimensionless number and a number with units. For example, if you are given a time as a sum of some numbers of hours, minutes, and seconds, it might be convenient to enter it in a form like this:

t = 3*(3600 s) + 47*(60 s) + (36 s)

The right hand side is a sum of three numerical values in seconds, so the result will be in seconds, which is what you want.