Elementary Algebra

$15\sqrt{21}$
First of all, in order to make this one large square root, we multiply the coefficients to obtain that the new coefficient is 15. Also, recall, $\sqrt{a}\times \sqrt{b}= \sqrt{a \times b}$. Thus, we can multiply 3 and 7 to obtain the simplified expression: $15\sqrt{3 \times 7}=15\sqrt{21}$