Which equation can be used to find the time, t, it takes for the ball to reach the ground?

Check the picture below, well that picture is using feet, but is pretty much the same curve for meters.
notice, it hits the ground when y = 0, or h(t) = 0, thus
[tex]\bf ~~~~~~\textit{initial velocity} \\\\ \begin{array}{llll} ~~~~~~\textit{in meters} \\\\ h(t) = -4.9t^2+v_ot+h_o \end{array} \quad \begin{cases} v_o=\stackrel{12}{\textit{initial velocity of the object}}\\\\ h_o=\stackrel{1.8}{\textit{initial height of the object}}\\\\ h=\stackrel{}{\textit{height of the object at "t" seconds}} \end{cases} \\\\\\ 0=-4.9t^2+12t+1.8\implies -4.9t^2+12t+1.8=0[/tex]