For items that are irregular in shape, it's difficult to use math to find out its volume. In this case, you can use water displacement to measure the volume of the item.
Step 1: Add a certain amount of water to a graduated cylinder that's big enough to fit the item you would like to measure. Record the initial volume of water by reading the bottom of the meniscus in the graduated cylinder.
Step 2: Carefully drop the item you're measuring down the graduated cylinder.
Step 3: Record the final volume of water by reading the bottom of the meniscus in the graduated cylinder.
Calculate the volume of the rock by showing your work and your answer on the image below.
Try this at your lab bench if you are in class.