Well, first, you need to document what you know:
$\displaystyle v_{y0} = 16 \frac{m}{s}$
$\displaystyle v_{yf} = ?$
$\displaystyle y_0 = 0 m$
$\displaystyle y_f = 7 m$
$\displaystyle t = ?$
$\displaystyle a = 9.8 \frac{m}{s^2}$
Now, you have everything you need to solve for t:
The easiest way to do this would be to find the final velocity at 7 m and then solve for time, the first equation we need is a basic kinematic equation:
$\displaystyle v_{yf}^2 = v_{y0}^2 + 2(a)(y_f)$
Plug in what we know:
$\displaystyle v_{yf}^2 = 16^2 + 2(9.8)(7)$
$\displaystyle = 256  137.2$
$\displaystyle = 119$
$\displaystyle v_{yf} = 10.9 \frac{m}{s}$
Now, we take another equation:
$\displaystyle v_{yf} = v_{y0} + at$
Plug what we know:
$\displaystyle 10.9 = 16  9.8t$
$\displaystyle 5.1 = 9.8t$
$\displaystyle 0.52 s = t$
And there you go.
