Answer:
Step-by-step explanation:
I will assume that you meant:  (-3/7)(y-28) - (1/21)y
Using the distributive property of multiplication, we get:
(-3/7)y + (3/7)(28) - (1/21)y
Let's first reudce the middle term.  We get:  12
Then we have:
(-3/7)y + 12 - (1/21)y
Here there are two different denominators.  We must combine the y terms.  The LCD here is 21.  Multiplying (-3/7)y by 21/21, we get the equivalent:
-9y
------ + 12 - (1/21)y
  21
or:
-9y          21
------ + 12----- - (1/21)y
  21           21
Combining the y terms, we get (-10/21)y:
-10y      252
------- + -------- = (1/21)(-10y + 252)
  21          21