Lời giải:

a) 4 * x + 15 = – 5 

    4 * x         = (– 5) – 15

    4 * x         = – 20 

         x         = (– 20) : 4 

          x        = – 5

Vậy x = – 5

b) (– 270) : x – 20 = 70 

  (– 270) : x            = 70 + 20 

   (– 270) : x           = 90 

                  x          = (– 270) : 90

                  x          = – 3

Vậy x = – 3