A rectangle is sometimes a quadrilateral

A rectangle is always a quadrilateral

Step-by-step explanation:

Quadrilaterals are shapes with 4 sides, a rectangle is a shape with 4 sides, if it did not have 4 sides, it would not be a rectangle. Same goes for all  quadrilaterals.

first, you divide the triangle in two, tô form a retangle triangle. after, using pythagoream theorem:

18^2 = 14^2 + (x/2)^2

(x^2/4) + 196 = 324

x^2/4 = 324 - 196

x^2/4 = 128

x^2 = 512

x = √512

x = √2^9

x = 16√2 cm

i hope i you.