Well of course you cannot plot (x-a)^2 directly without specifying a value for a since it is not numerical with an arbitrary value of a. You can plot a two-dimensional plot varying both x and a line this:
Plot3D[(x - a)^2, {x, -2, 2}, {a, -1, 1}]
Or you could look at how a plot of (x-a)^2 varies as you change a dynamically using Manipulate, for example:
Manipulate[Plot[(x - a)^2, {x, -2, 2}], {a, -1, 1}]
And, of course, there are many other possibilities...
