Determine \(t_4 \) and \(t_7 \) of a geometric sequence with \(t_1 = 15\) and \(r = -\frac{1}{3}\).  Leave your answers as fractions.
    \[\begin{align}
     t_n &= t_1 r^{n - 1}  \\
     t_4 &= 15\left( { - \frac{1}{3}} \right)^{4 - 1}  \\
     t_4 &= -\frac{5}{9} \\
     \end{align}\]
    \[\begin{align}
     t_n &= t_1 r^{n - 1}  \\
     t_7 &= 15\left( { -\frac{1}{3}} \right)^{7 - 1}  \\
     t_7 &= \frac{5}{{243}} \\
     \end{align} \]