Recall that int returns the largest integer less than or equal to the number inside it, in this case 2x. To put it more simply, it rounds down to the nearest integer.
For x=-1.8
f(-1.8)=int(2*-1.8)
f(-1.8)=int(-3.6)
The integer less than -3.6 is -4 so:
f(-1.8)=-4
Result:
-4