Could someone explain this to me? i really need assistance or i may fail this year. if you in anyway i will be so grateful.. you!

Well, we have to solve for x.
So here's what we know:
10+x=A
60=B
120-x=ACD
In a triangle we know that the insides of a triangle have to = 180, we already know that b is 60 degrees that leaves us with 120 degrees, from here it is just guesswork, what can we add 10 to, and get a number less than 120, but take it and 120 and subtract them to get our angle that we need ACB.