Respuesta:
1) x = 12
2) a = 11
3) c = 7
Explicación paso a paso:
=> x + 5 = 17
x = 17 - 5
x = 12
=> 15 - a = 4
15 - 4 = a
11 = a
=> 4x = 28
x = 28/4
x = 7