Take any value of x and see how many times fun1 is called. y doesn't matter here.
If x=4 then fun1(4,y) --> fun1(3,4+y) -->fun1(2,3+y)--->fun1(1,2+y)--->fun1(0,1+y)
5 times.
Similarly for any value of x fun1 will be called for x+1 times and in each call only constant work is done like comparison. So O(c*(x+1))=O(x)