Its a simple equatioon,
First write the information that is given to you (as an equation) 14+x=17
Since you have a variable that is "x" you need to take it out of the equation as x=17-14 and put the 17 (that is the result) and then since you're adding 14, at the other side you'll substract it, finally do the opperation and you get your result that is x=3