To flip this shape across the line of reflection:
1. Start with one vertex. Count the number of units from the vertex to the line of reflection. (In this shape, vertex A is 4 units over from the line of reflection.)

2. Starting at that point on the line of reflection, count the same number of units away from the line. (In this shape, count 4 units up from the line of reflection.) Place the new vertex here. 

3. Do the same with the other vertices. 
Vertex B is 4 units over from the line. Count 4 units up from the line and place the new vertex here.
Vertex C is 1 unit over from the line. Count 1 unit up from the line and place the new vertex here.
Vertex D is 1 units over from the line. Count 1 unit up from the line and place the new vertex here.
 
4. Draw the lines to connect each vertex and finish the shape.