Error when installing .net framework 3.5

You'll need your install media for this!

At an elevated command prompt, run the following command:
Dism /online /enable-feature /featurename:NetFx3 /All /Source:<drive>:\sources\sxs /LimitAccess

Note In this command, <drive> is a placeholder for the drive letter for the DVD drive or for the Windows 8 installation media or a shared folder containing the files.

For example, you run the following command:
Dism /online /enable-feature /featurename:NetFx3 /All /Source:D:\sources\sxs /LimitAccess