Here's another method :
Length of a side = sqrt(6^2 + 18^2) = sqrt (36 + 324) = sqrt (360)
And using the Law of Cosines, we can find the smaller interior angle of the rhombus
12^2 = 360 + 360 - 2(360)cos(theta)
[144 - 360 - 360] / [ -2(360)] = cos (theta)
cos-1 [ [144 - 360 - 360] / [ -2(360] ] = theta = 36.869897645844°
Using the Law of Sines.....we can find the height - h - as follows :
sqrt(360) = h / sin(36.869897645844°)
h = sqrt(360)* sin (36.869897645844°) = 11.3841995766061656 cm
Here's a pic : [DE is the height]

