The steps below show the incomplete solution to find the value of x for the equation
6x − 2x − 5 = −6 + 21:

Step 1: 6x − 2x − 5 = −6 + 21
Step 2: 6x − 2x − 5 = 15
Step 3: 4x − 5 = 15

Which of these is most likely the next step?

4x = 10
4x = 3
4x = 45
4x = 20