What is the unknown number? which property did u use? (_+8)+32=49

First of all, subtract 32 from both sides:
(?+8)+32-32=49-32
?+8=17
And then, we subtract 8 from both sides again:
?+8-8=17-8
?=9
So the unknown number is 9. 
Hope this helps! :D I'm not sure what type of property the question was looking for, so can't really help you there aaa sorry ;w; But it probably is the Subtraction Property of Equality (just looked it up, but I'm not sure if that's the answer you were looking for)~
(I'm using ? because I'm assuming that you are not familiar with using x or other letters as the unknown so yeah :D)

Do the inverse operation