In step 2 multiply what is inside the parenthesis by 7 (distributive property):
7 * x = 7x
7 * 3 = 21
In step 3 subtract -7x on both sides of the equation. (Property of subtraction of equality)
Then -7x + 4x = -3x
7x-7x + 21 = 21
In step 4, both sides of the equation are divided by -3. (owned by the equality division)
So:
-3x / -3 = x
-21 / -3 = 7
Recall that the division of equal signs results in a positive number.