Add a radical to itself if the radicands are alike. This will double the radical. For example, you can add √x + √x, but you can't add √x + √y. When you have like radicands, keep the radicand the same and add the coefficients. The coefficients are the numbers to the left of the radicals. If there is no number to the left of a radical, its coefficient is 1. For instance, √x + √x equals 2√x. If you add 2√a + 2√a, you get 4√a.
Multiply a radical times 2 to double it. For example, if you have √y, you can multiply it times 2 to get 2√y. Again, you actually multiply two times the coefficient of the radical.
Convert radicals that aren't square roots to fractions before doubling. For instance, you might have ^3√x. The small number to the upper left of the radical sign is the index. Place the radicand's exponent in the numerator of a fraction (top portion) and the index in the denominator (bottom part) of the fraction. Raise the radicand to this power; if the radicand has no written exponent, its exponent is 1. Therefore, ^3√x would become x^1/3. Then multiply the fraction times 2/1 to double it; x^1/3 becomes x^2/3.