The area of a triangle of base b and height h is:
[tex]A=\frac{b\cdot h}{2}[/tex]The area of the shaded region can be calculated as the difference between the bigger triangle of dimensions 4 x 14 units and the smaller triangle of dimensions 2 x 7 units.
Recall the dimensions of the smaller triangle are half the dimensions of the bigger triangle because A and B are the midpoints of their respective segments.
The area of the bigger triangle is:
[tex]A_b=\frac{14\cdot4}{2}=28\text{ square units}[/tex]The area of the smaller triangle is:
[tex]A_s=\frac{7\cdot2}{2}=7\text{ square units}[/tex]The area of the shaded region is:
[tex]\begin{gathered} A=28\text{ square units }-7\text{ square units} \\ \\ A=21\text{ square units} \end{gathered}[/tex]