What is the area in square units of triangle QRS also show work

Find the area of the larger rectangle. 
5 x 4 = 20  
One of the triangles is 2 x 2. Area of 2. Subtract 2 from 20.
20 - 2 = 18 
Another is 2 x 5. Area of 5. Subtract 5 from 18. 
18 - 5 = 13 
The last is 3 x 4. Area of 6. Subtract 6 from 13. 
13 - 6 = 7 
The area of the triangle QRS is A, 7. 
Hope this helps!