4x+21=7 Please note this problem should be worked under a sqare root sign?

Your question is a little ambiguous
If you mean
4x +21 = 7
then
4x = -14
x = -3.5

If you mean √(4x+21) = 7
Then 4x +21 = 7² --> square both sides
4x = 49 - 28 = 21
x = 7

4x + 21 = 7
- 21 -21
4x = -14
/4 /4
x = -3.5

check
4 x 3.5 + 21 =7
-14 + 21

A mind reader would guess at :-

√ ( 4x + 21 ) = 7

4x + 21 = 49

4x = 28

x = 7

You have to be more clear. Is the problem:
sqrt(4x) +sqrt(21)=7
sqrt(4x+21)=7
sqrt(4x+21)=sqrt(7)
something else???

4x + 21 = 7
4x = 7 - 21
x = -14/4
x = -7/2 (-3.5)