Question

fill in the other coordinate for the line 7x - 5y =21:(4 , ?)

Answer 1

7x-5y=21 when x=4 from the coordinate point of (4,y)
Next,
7x-5y=21
Substitute x with 4
7(4)-5y=21
28-5y=21
Add 5 to each side
28-5y+5y=21+5y
28=21+5y
Subtract 21 to each side
28-21=21+5y-21
7=5y
Divided 5 to each side
7/5=5y/5
y=7/5
Or
y=1 2/5
Check:
7x-5y=21
Substitute x with 4 and y with 7/5
7(4)-5(7/5)=21
28-7=21
21=21. As a result, (4,7/5) is your correct answer. Hope it help!

Answer 2

If you know that x is 4, (assuming it's placed as x,y) then simply substitute 7(4)-5y=21.
28 - 5y =21.
28 - 21 =5y
5y= 5
Y= 1
Not sure if I'm right but I think that's how it's done the simple way.