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.