Factor $63=3^{2}\times 7$. Rewrite the square root of the product $\sqrt{3^{2}\times 7}$ as the product of square roots $\sqrt{3^{2}}\sqrt{7}$. Take the square root of $3^{2}$.
$$3\sqrt{7}-\sqrt{175}+\sqrt{28}$$
Factor $175=5^{2}\times 7$. Rewrite the square root of the product $\sqrt{5^{2}\times 7}$ as the product of square roots $\sqrt{5^{2}}\sqrt{7}$. Take the square root of $5^{2}$.
$$3\sqrt{7}-5\sqrt{7}+\sqrt{28}$$
Combine $3\sqrt{7}$ and $-5\sqrt{7}$ to get $-2\sqrt{7}$.
$$-2\sqrt{7}+\sqrt{28}$$
Factor $28=2^{2}\times 7$. Rewrite the square root of the product $\sqrt{2^{2}\times 7}$ as the product of square roots $\sqrt{2^{2}}\sqrt{7}$. Take the square root of $2^{2}$.
