The $4^{th}$  term of a $G.P$  is square of its second term and the first term is $-3$ , then its $7^{th}$  term

$a_4=a{r}^3=\left(-3\right)r^3$

$a_2=a{r}^1=\left(-3\right)r$

According  to the  problem

$a_4={\left(a_2\right)}^2$

$-3r^3={\left(-3r\right)}^2$

$-3r^3=9r^2$

$r=-3$

$a_7=a{r}^6=\left(-3\right){\left(-3\right)}^6={\left(-3\right)}^7=-3^7$