2x+3=5x-42 what is the number that belongs in the green box

Objective: Solve 2x + 3 = 5x - 42 for x.

Step 1: Subtract 3 from both sides.

2x + 3 = 5x - 42

2x = 5x - 45

Step 2: Subtract 5x from both sides.

2x = 5x - 45

-3x = - 45

Step 4: Divide both sides by - 3.

-3x = - 45

x = 15

Answer:

x = 15