From the figure attached,
Coordinates of the vertices of the given rectangle ABCD,
A(3, 6), B(12, 6), C(12, 1) and D(3, 1)
Rule for the reflection of a point across x-axis,
(x, y) → (x, -y)
After reflection of the given rectangle across x-axis image points will be,
A(3, 6) → A'(3, -6)
B(12, 6) → B'(12, -6)
C(12, 1) → C'(12, -1)
D(3, 1) → D'(3, -1)
Now we can plot these points on the graph to get the image of rectangle ABCD.