question about geometric sequence problem

i need help this this question. in a geometric sequence the second term is 28 and the fifth term is 1792. find the 8th term.

$$Formula: \boxed{~ a_n=a\cdot r^{n-1} ~}$$

$$a_2=28=a\cdot{r}^1 \qquad a_5 = 1792=a\cdot r^4\qquad a_8 = a\cdot r^7$$

$$\dfrac{a_5}{a_2}=\dfrac{a\cdot r^4}{a\cdot r^1} = r^3\\\\\\ \small{\text{$ \begin{array}{rcl} r^3 &=& \dfrac {1792}{28}\\\\ r &=& \sqrt[3]{\dfrac {1792}{28}}\\\\ r&=&\sqrt[3]{64}\\\\ r &=&4 \end{array} $}}$$

$$a=\dfrac{28}{r}=\dfrac{28}{4}=7$$

$$\\a_8 = a\cdot r^7\\ a_8= 7\cdot 4^7\\ a_8 = 7\cdot 16384\\ a_8 = 114688$$