The solution is simple 11*5/2. 11= the base or bottom of the triangle. 5=the high or side. the triangle is 1/2 of a rectangle so divide by 2. the solution is 27.5 units.
Just count the number of squares:
(9+5)*2=14*2=28
I think the correct answer from the choices listed above is option A. From the image given above, the steps made by the students were all correct and she was able to solve for the value of c. Hope this answers the question. Have a nice day.

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Hope you could get an idea from here.
Doubt clarification - use comment section.