63x
The amount of magnification you see through a microscope is extremely easy to calculate. You just multiply the power of each lens the image travels through together. So you have the equation
630 = 10 * x
where 630 is the total magnification and 10 is the magnification of the ocular lens. Now just solve for x by dividing both sides by 10
63 = x
So the objective lens has to have a power of 63x for your total magnification to be 630x
