Question 87816
AN EQUILATERAL TRIANGLE HAS THREE EQUAL SIDES. tHUS 18/3=6 IS THE LENGTH OF EACH SIDE.
THE ALTITUDE CUTS THE BASE IN HALF SO NOW WE HAVE A RIGHT TRIANGLE WITH THE BASE=3 & THE HYPOTENUSE=6. 
NOW WE SOLVE FOR THE ALTITUDE.
3^2+A^2=6^2
9+A^2=36
A^2=36-9
A^2=36-9
A^2=27
A=SQRT27
A=5.196 ANSWER.