r . . V w l U i. a v w. v . Una It 3 A 1-
\(7x^2 + 21 = 0\\\\7x^2 = -21\\\\x^2 = -\frac{21}{7}\\\\x^2 = -3\\\\x = \sqrt{-3}\\\\\boxed{\bf{x = i\sqrt{3}}}\\\\\boxed{\bf{x=-i\sqrt{3}}}\)
