You can't.
17 + 2n = 21 + 2n gives: 2n = 4 + 2n which gives 0=4 which is not possible so the sum is not solveable.
17 plus 21 equals 38
A + 9 = 21 A = 21 - 9 A = 12
5
-3+y=17+1 im assuming solve for y?-3+y=18 add 3 to both sidesy=21
depends what 'a' =
21 Uhm, no, 17 actually...
-21
3+4m+5m=214m+5m=21-39m=18m=18/9m=2
21+21+17=59
2
2z+17=21 subtract 17 from both sides. 2z=4 divide both sides by 2 z=2
13 and 17/21