the equation of two sides of a parallelogram are $2x-3y+7=0$ and $4x+y-21=0$ and one vertex is $(-1,-3)$. Find the other vertices. First, I checked if the point $(-1,-3)$ is not a solution to the two given equations above so therefore none of those lines passes that point. Then, I solved for the lines parallel to equations above getting $2x-3y-7=0$ and $4x+y+7=0$ respectively.
Edit: i managed to find the other vertices which are (5,1), (4,5), and (-2,1). I just want to ask if my vertices are correct.
 A: You just need to intersect each of the sides to obtain the vertices. For instance, from $2x-3y-7=0$ and $4x+y+7=0$ you have the point that you already know, $(-1,-3)$:
$$\begin{cases}2x-3y-7=0 \\ 4x+y+7=0\end{cases}\quad \Rightarrow\quad x=-1,y=-3.$$
Try with the other intersecting pairs of lines:


*

*Second vertex: $2x-3y-7=0$ and $4x+y-21=0$

*Third vertex: $2x-3y+7=0$ and $4x+y-21=0$

*Fourth vertex: $2x-3y+7=0$ and $4x+y+7=0$


Solution:

 The vertices are $(-1,-3)$, $(-2,1)$, $(4,5)$ and $(5,1)$.

A: The parallelogram has $4$ vertices. The vertex $(-1,-3)$ is not an element of both given equations. The $2$ equations given are not parallel to eachother.
This tells us that the equations are from 2 abutting sides. These equations can only obtain $3$ vertices, so the given vertex is the $4^{th}$ vertex. You've already solved it almost. You calculated the equations of the other $2$ sides.
By calculating the $3$ remaining intersection points of the non-parallel equations you've found your other vertices. 
