.net framework error in sql 2008r2

You added Roles and Features. So now in Features, does .NET show up as Checked (Installed).

Download .NET Framework Setup Verifier and check the .NET installs

http://blogs.msdn.com/b/astebner/archive/2008/10/13/8999004.aspx

Try to install it again with this command. Not that d:\sources\sxs should be your installation media

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs

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>> It requires dotnetframework 3.5

To be more exact it is .Net Framework 3.5 SP1 and not just 3.5
SQL Server 2008 R2 installer requires the below software to be installed prior if you are trying to install Express edition of SQL Server 2008 R2.
Other editions of SQL Server will install .Net Framework 3.5 SP1 by default..
Also ensure that you are having the below softwares installed before running SQL Server 2008 R2 installer.
https://msdn.microsoft.com/it-it/library/ms143506(v=sql.105).aspx

>> However I added roles & features for dotnet.

If you have installed .Net Framework 3.5, then restart the Server before running SQL Server 2008 R2 installer..

>> I m trying to Install SQL 2008r2 on newly bulit Windows Server20088r2

Just curious, why are you still using SQL Server 2008 R2 when the product support is already over, try to use SQL Server 2016/2017..