No need to be confused friend. It does seem confusing but all you really need to remember is that a special property of rectangles which is that their sides always meet at a 90 degree angle. For this problem you are given two angles ADB and CDB that together make up a corner meaning that added together, they should add up to 90 degree angle. You are given the angle in the form of an equation, so all you have to do is add them both up and equate them to 90 as such:
90=(ADB) + (CDB)
90=(2y+40) + (-3y+51)
90=y+91
solving for y, you get y=1.
therefore to find the angle CBD, you just have to note that CBD is equivalent to ADB. and plug in y=1 to 2y+40 to get 42.
