For the cylinder on the left-hand-side,

now, for the triangular prism on the right-hand-side,
notice is really just 2 triangles and 3 rectangles, stacked up to each other at the edges.
the triangles have a base of 4, and a height of 1.5.
the rectangle on the left and the one on the right is a 6x2.5 rectangle.
the rectangle at the bottom, is a 4x6 rectangle.
adding their areas, is the area of the prism,
D, because all of the others when multiplied are greater or less than 34. If you multiply the sides of D you get 34 exactly.
Less than 5 is your answer
First, divide by a to get c + b = d/a. Next, subtract the b to get c = d/a - b.
Optional:
c = d/a - ba/a = (d-ba)/a
The answer would be .67 or .66666... since it’s infinite; or 2/3 in fraction form