Hey there! I'm happy to help!
An irrational number is a real number that can't be written as a ratio of two other integers (non-fraction numbers) so a number like π or √2.
An imaginary number is one that breaks a mathematical rule so it can't necessarily exist as a real number. The main imaginary number is i, and here is the thing it does that breaks the rules and makes it imaginary:
i²=-1
so
√-1=i
We know that you cannot square a real number and have it be negative because a negative number can only be obtained by multiplying a positive and a negative number. This also shows us that there cannot be a square root of a negative number.
We have the number -√15. The placement of that negative sign is CRUCIAL! The square root of any positive integer is going to be a real number, but the square root of a negative number is nonexistent aka imaginary. √-15 would be an imaginary number, but we have -√15, which is the same thing as -1·√15. The negative sign is not inside the square root, which makes it not imaginary, so this is an irrational number. You can plug it into a calculator and it would be -3.872983346....
Have a wonderful day and keep on learning :)