You can use the matlab command readmatrix to get the data in from Excel (type doc readmatrix on the command line for details)
This will return all the values in your spreadsheet, not just the ones where x > 200. However if you have now put these into a variable, let's call it x, you can get just those values using something like:
x = x(x>200)
to evaluate the expression you have just use
y = 3*x + 12
Just as you already have it written
