How do you expand video memory in a virtual machine?

I have just installed Ubuntu 14.04 on VirtualBox, including the guest additions.

But the display is really slow Looking at the display settings i can see that the video memory is only 12mb but i can't move the slider to increase it. The checks for extended features are also greyed out Check the screenshot

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Tell me the best way to increase memory?

TIA

Virtualbox supports up to 256 mb of video ram This cannot be set using the virtual box manager's slider

To use the full use of all the supported memory we can issue the following command in a terminal

VBoxManage modifyvm "Name of VM" --vram 256

Before we change settings such as video ram a preexisting virtual machine must be shut down

Note that for 3D video hardware acceleration from the guest addition's video driver the physical RAM of the host graphics card will be passed through. This will not affect the ram video settings of the virtual machine