Step 1. None needed. This is the original expression
Step 2. Distributive property
Step 3. Multiply (the 3 and 5 multiply to 15)
Step 4. Commutative property. Specifically, this is the commutative property of addition which allows us to add any two numbers in any order we want. In this case, the -15 and 4x terms swap places.
Step 5. Associative property. The parenthesis group the first two terms away from the third term.
Step 6. Combine like terms. Both 3x and 4x have an x only so they are like terms that combine to 7x. You can think of it as 3+4 = 7, then you just tack an x onto everything to get 3x+4x = 7x.
