How do we practice writing and simplifying expressions with units in mind

In simplifying variables and coefficients, you just have to add or subtract the coefficients with the same terms. The terms are the same when they both have the variable x and exponent.

Example 1:

You can simply add 5 and 6 and copy variable x^2 because they are similar terms.

But for expression, coefficients 4 and 6 cannot be added because they are not the same terms.

Example 2:

Simplify