The problem describes an arrowhead and a parallelogram joined together. An equilateral triangle is formed using one of the parallelogram's diagonals. We need to calculate the size of angle y.

Step 1: Analyze the given information and diagrams.
The problem describes an arrowhead and a parallelogram joined together. An equilateral triangle is formed using one of the parallelogram's diagonals. We need to calculate the size of angle $y$.

Let's label the vertices of the arrowhead as $A, B, C, D$, where $A$ is the top vertex, $B$ and $D$ are the "shoulders", and $C$ is the concave vertex.
Let the parallelogram be $ADEF$.
From the diagrams:

• The arrowhead has two equal sides, $AB$ and $AD$, indicated by single dashes. Let their length be $s$.
• The parallelogram is joined to the arrowhead along side $AD$. Thus, $AD = s