Can you guys help me with this i don't get it​

What is the missing reason in step 5? linear pair postulategivendefinition of complementary anglescongruent complements theorem

Follow these steps using the algebra tiles to solve the equation −5x + (−2) = −2x + 4. 1. add 5 positive x-tiles to both sides and create zero pairs. 2. add 4 negative unit tiles to both sides and create zero pairs. 3. divide the unit tiles evenly among the x-tiles. x =

Atriangle has a side lengths of 18cm, 80 cm and 81cm. classify it as acute obtuse or right?