It is simple: Keep all the unknowns (x 's) on the Left Hand Side (LHS) and move all the known to the Right Hand Side (RHS), so we have:
8x = -16
or x = -2.
now check your answer, by substituting the value of x in the first equation:
3x +15+5x = -1
The key to enjoying any subject you take in School or College is simply to know how the subject is or will be applicable in your daily life and possible future career!
If you can challenge yourself, your teacher/professor and family/friends to show you the meaning of learning a subject, by actually showing why and how it is important as part of your daily life or career in the future profession...