Generate a random number with max, min and mean (average) in visual basics - triangular dist

I am looking to see how I can create a triangular distribution using 3 numbers of minimum (40) maximum (330) and average (100).

this code just give me the random between 2 numbers and does not carry the average :

 z = =RAND()*(b-a)+a

How can I make it a triangular dist. ?