Because you cannot combine terms that are not alike.
2a+5a+9a = 16a
If: 2a+13 = 5a+1 Then: a = 4
-7a+2a+9 is also -5a+9. -5a+9=24. -5a=15. a=-3.
2a + 3a + 4 = 5a + 4
5a-20+2a = 6a-11 5a+2a-6a = 20-11 1a = 9 a = 9
2a + 3a = 5a
5a + 3 + (2a - 2) + a= 5a + 3 + 2a - 2 + a= 8a + 1
If 3a+5=5a+3, then: 3a+5 - 3a = 5a+3 - 3a 5=2a+3 5 - 3=2a+3 - 3 2=2a a=1
2a + 5a + 3a = 10a
7a + 4 = 5a + 7 7a = 5a + 3 2a = 3 a = 3/2
2a-6a-a=b+4b-4b -5a=5b-4b -5a=1b a=-5 b=1
2a + 10 + 5a - 2 = (2a + 5a) + (10 - 2) = 7a + 8