-16u + 13u – -18u + 5u = -20

Think of negative numbers as cold numbers and positive numbers as hot numbers (like with temperature)

If you take away cold then it gets hotter. So --=+

If you add heat it gets hotter So ++=+

If you add cold it gets colder. So +-=-

and if you take away heat it gets colder so -+=-

$$-16u+13u--18u+5u=-20\\\\
-16u+13u+18u+5u=-20\\\\
(-16+13+18+5)u=-20\\\\
20u=-20\\\\
u=-1$$