Answer: 13
Explanation:
 16-x=3
 16-x-3=3-3 ⇔ subtraction property of equality
 13-x=0
13-x-13=0-13 ⇔ subtraction property of equality
 -x=-13
 x=13 
-----------------------------------
Easier way if you know how to do equation
16-x=3
 -x=-13 (skip all the subtracting parts)
 x=13
Hope this helps!! :)