Pentagons and hexagons can be irregular.
Anything that does not have congruent sides is irregular.
Hope that helped :))
The first thing you should do is use the fact that the area of the triangle is:
A = b * h * (1/2).
From there you clear the height as a function of "c"
Then you will have a polynomial division which you must make to find the height of the triangle.
I attach the solution.
9 are left take 17 and subtract 8