Hello!
As we have the command DISPLAY (a) in the end, and a ← 3, it means that the variable a will receive the value 3.
After that, we have another attribution:
b ← 15, that means b will receive the value 15.
So, at this moment we have these values:
a = 3
b = 15
But now we have the last attribution:
the variable a will receive the value contained in b, expressed by a ← b, so the first value of a will be superscript by the value of b, resulting in:
a = 15
b = 15
So, the program will display the value of a, 15.