Sum 81 to both sides to get
[tex]4x^2 = 81 [/tex]
Divide both sides by 4 to get
[tex] x^2 = \frac{81}{4} = \frac{9^2}{2^2} [/tex]
Consider the square root of both sides. Note that this requires the [tex] \pm [/tex] sign, since [tex] a^2=b^2 [/tex] implies [tex] a=b [/tex], but also [tex] a=-b [/tex]. So, the solutions are
[tex]x = \pm\sqrt{\frac{9^2}{2^2}} = \pm\frac{9}{2} [/tex]
