Since BE and CD are parallel, triangles ABE and ACD are similar, meaning their side lengths are proportional.
AC = AB + BC = 1 + 3.5 = 4.5
The proportion of AB to AC (corresponding sides of two similar triangles) is 1 / 4.5
Let x be the variable that represents the unknown length of AE
The proportion of AE to AD (another set of corresponding sides of two similar triangles) is x / 18
Since the triangles are similar, these two proportions must be equal.
1 / 4.5 = x / 18
Cross multiply
4.5x = 18
Divide both side by 4.5
x = 4
The length of AE is 4, no need to round since the answer is already a whole number.
Let me know if you need any clarifications, thanks!