on touch - object set object bullet angle of motion to: angle(object.x,object.y,touch.x,touch.y) it should work as wanted..
If the object isn't moving yet you will have to set the bullet-speed too ( it has to be set before setting the angle if I recall right)
This site has a good manual and many tutorials, I'd recommend reading them to get better grips on working with events and expressions.
Even if they are not for the exact purposes you want, there is still a lot to learn from them regarding working with construct2 in general.
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..