Block formulas are surrounded by double dollar signs. For example, $$x = \frac{-b \pm \sqrt{b^2 - 4ac} }{2a}$$ renders, on a separate line, as

x = \frac{- b \pm \sqrt{b^{2} - 4 a c}}{2 a} .

$x = \frac{-b \pm \sqrt{b^2 - 4ac} }{2a}.$