Question: How can I write integration symbol?
You can't! Not on this site I'm afraid.
If you look on the menu bar above your text, when you are entering a problem, you will see an Omega symbol. (The Greek letter omega)
Clicking on this symbol will open up a window from which you can choose special symbols. The integration symbol isn't among them.
If you wish to enter an integration problem, in which, say, you wanted to integrate the function f(x), with respect to x, between the limits of a (lower limit) and b (upper limit), then you would enter it like this.
int [ a to b] f(x) dx
or,
int [ a,b] f(x) dx
HTH