Hey there!
Let's solve all these equations.
The first equation:
x+7=12
subtract 7 from both sides:
x=5
not 15 ;)
The second equation:
15+x=10
subtract 15 from both sides:
x=-5
not 15 ;)
13+x=38
subtract 13 from both sides:
x=38-13
x=25
not 15 ;)
let's try the last option (it has to work)
x+24=39
subtract 24 from both sides:
x=15
Yep!
Hence,
Hope everything is clear.
Let me know if you have any questions!
#LearningDoesn'tEnd
:-)