How can i write the formula for strontium fluoride?

First thing, you need to know what this molecule is made of. 
The First atom is the Strontium Sr (For more information, used usually as a substitute of the calcium in the human body).
The Second Atom is made of 2 Fluoride F2 (because it chemical name is Strontium difluoride).
Then, you join them together and Get 
And for more information, Strontium fluoride is usually used as an optical material.

Hope this Helps :)

The answer is  a: x-rays—2 and ultraviolet rays—3.  hope i !