c2 differentiation symbols?

x E R means that "x is any real number" i.e. that x^2 + 12 > 0 for any number that you can name, be it negative, a fraction, or something like root 2 or pi which are irrational numbers.

x^2 + 12 - well, x^2 will always be greater than or equal to zero (if you square a number then you always get a positive number, the smallest possible number you can get is 0). then if you add 12 to that you will always get a number that is bigger than 0 (you'll always get a result that is bigger than or equal to 12).
There are weird things involving the square roots of negative numbers, which are not called real numbers, and that's where the "x E R" bit comes from.

love danniella

Or more simply you rearrange to get x²>-4/5 which you know is always true for all values of x on the Real number line.

Out of interested, how did you get that ℝ symbol? It doesn't appear to be in charmap and Opera (my browser) normally dislikes non-standard characters.