Simplify the square root
[tex]7 \sqrt{90} = 7\sqrt{3 { }^{2} \times 10} [/tex]
Separate the numbers in the square root to look like this
[tex]7 \sqrt{3 ^{2} } \sqrt{10} [/tex]
Because three is squared, it cancels out the square root
[tex]7 \times 3 \sqrt{10} [/tex]
Simplify to get your answer:
[tex]21 \sqrt{10} [/tex]
