Answer:
B
Step-by-step explanation:
A. these simply to 21 + 7x and 21 + 3x
- this one is incorrect because the 7x and 3x are different
B. these simplify to 10 + x and 10 + x
- this is the correct one since they are exactly the same
C. these simplify to 21x and 7x + 21
- these two are not the same because one expression has only a number with a variable but the other one has a number with a variable <em>and </em>another number
D. these simplify to and
- in subtraction, the order of the 3 and the x matters, so the equations are not the same
- example: 6 - 4 = 2 but 4 - 6 = -2