In the figure below, PQ and QR are drawn to scale.

1. Identify the properties of a parallelogram to locate point S

In a parallelogram $PQRS$, the opposite sides are parallel and equal in length. Therefore, the vector $\vec{PS}$ must be equal to the vector $\vec{QR}$, and $\vec{SR}$ must be equal to $\vec{PQ}$.

2. Locate point S using grid movements

From point $Q$ to point $R$, count the number of grid units horizontally. Since $Q$ and $R$ are on the same horizontal line, $\vec{QR}$ is purely a horizontal translation to the right. Apply this exact same horizontal translation starting from point $P$ to locate point $S$.

3. Draw the required lines

Draw a straight line segment from $P$ to $S$, and another straight line segment from $S$ to $R$.