add 3√21 + 7√21.

To add 3√21 + 7√21, we simply combine the coefficients (numbers in front of the square roots) while keeping the same root (√21).

The coefficient of the first term is 3.
The coefficient of the second term is 7.

By adding the coefficients:
3 + 7 = 10

Therefore, the sum of 3√21 + 7√21 is 10√21.