MBa(OH)2 = ABa + 2AO + 2AH = 137 + 2*16 + 2*1 = 171 (this is the molecular mass for barium hydroxide)

In 171 grams of Ba(OH)2 ..........there are...........137 grams of Ba
In x grams of Ba(OH)2...........there are...........3.94 grams of Ba

From now on I think you can find the quantity of Ba(OH)2

Good luck!