`7(sqrt(m+1)-3) = 21`

`sqrt(m+1)-3 = 21/7`

`sqrt(m+1)-3 = 3`

`sqrt(m+1) = 6`

Get the power of 2 in both sides.

`(sqrt(m+1))^2 = 6^2`

`m+1 = 36`

` m = 35`

*So the answer is m = 35.*

`7(sqrt(m+1)-3) = 21`

`sqrt(m+1)-3 = 21/7`

`sqrt(m+1)-3 = 3`

`sqrt(m+1) = 6`

Get the power of 2 in both sides.

`(sqrt(m+1))^2 = 6^2`

`m+1 = 36`

` m = 35`

*So the answer is m = 35.*