we know that
The area of a trapezoid is equal to
A=(b1+b2)h/2
where
b1 and b2 are the parallel bases
h is the height
In this problem we have
b1=8 cm
b2=4 cm
h=x
substitute
A=(8+4)x/2
A=6x cm^2
we know that
the area is less than 18 cm^2
so
6x < 18
solve for x
divide by 6 both sides
x < 3 cm
therefore
The value of x must be less than 3 cm