Bonsoir

A =  (5 / - 42 ) : ( 5/56 )

  = - 5/42 x 56/5

  = - 280/210

  = - 4/3

B = 10 : 10 ⁻² = 10 ¹

N =  3 x 10 ⁻⁶ x 7 x 10 ⁻⁹ x 2 x 10 ⁹

N = 42 x 10 ⁻⁶

C = 0.0071 x10 ³ = 7.1 x 10 ⁻³ x 10 ³ = 7.1 x 10 ⁰ = 7.1