What's happening here?

Question

int x = 10;
x += 4; // equivalent to x = x + 4
x -= 5; // equivalent to x = x - 5

how?pls...

Answer 1

As Freezemage wrote is simple.

In the first case x+=4 means that x will have a new value which will be x+4 and of course
x=x+4=10+4=14

In second case in the same way new x will be x-5 so
x=x-5=14-5=9

Answer 2

To make it more simple, look at it that way:
x += 4 -> x + 4 = x
x -= 5 -> x - 5 = x