Solve for b
b=\sqrt{3}\approx 1.732050808
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\sqrt{21}b=3\sqrt{7}
Swap sides so that all variable terms are on the left hand side.
\frac{\sqrt{21}b}{\sqrt{21}}=\frac{3\sqrt{7}}{\sqrt{21}}
Divide both sides by \sqrt{21}.
b=\frac{3\sqrt{7}}{\sqrt{21}}
Dividing by \sqrt{21} undoes the multiplication by \sqrt{21}.
b=\sqrt{3}
Divide 3\sqrt{7} by \sqrt{21}.
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