It's theoretically simply a matter of solving the equation, but not really in this case:
u - 9 = u + 7
First get all constants (regular numbers) on one side and all variables on the other:
u = u + 7 + 9 (add 9 to both sides)
u = u + 16 (simplify)
^ To get rid of this 'u', we must subtract 'u' from both sides:
u - u = 16 (subtract u from both sides)
0 = 16 (simplify -- any number minus itself is zero)
0 never equals 16!
Therefore, there is no value that could possibly be substituted in for u that would make the statement true.
Don't believe me? Try it:
What if u = 2?
2 - 9 = 2 + 7
-7 = 9
-7 NEVER equals 9!
Here's a shorter way to do it:
Take the original equation
u - 9 = u + 7
and subtract u from both sides.
-9 = 7
-9 NEVER equals 7!
