What is the complex conjugate of 210?

Simply alter the sign of the imaginary part to find a complex conjugate (the part with the i). It either goes from positive to negative or from negative to positive, according to this.
As a general rule, the complex conjugate of ${\displaystyle a+bi\text{is}a-bi}$.
You make an odd argument. There is no imaginary component in your number. Therefore, ${\displaystyle 2\sqrt{10}}$, if expressed as a complex number, would be written as ${\displaystyle 2\sqrt{10}+0i}$.
Thus, the complex conjugate of ${\displaystyle 2\sqrt{10}+0i\text{is}2\sqrt{10}-0i}$, which is still equal to ${\displaystyle 2\sqrt{10}}$.