Its C because the 18in is the height, the 12in is the width, the 24in is the length. Which you just multiply together.
We have measures of all tree sides of the both triangles,
so we can use SSS to check if the triangles are similar.
|ED|/|AB| =5/10=1/2
|DC|/|BC| = 4/8 = 1/2
|EC|/|AC| = 6/12 = 1/2
We see that all tree pairs are in proportion, so these triangles ΔABC and ΔEDC are similar.
We have enough information to prove that ΔABC similar to ΔEDC.
43.9 + n = 49.65
43.9-43.9 + n = 49.65 -43.9
n = 5.75