is methane mass should be burned to produce 12,880 kj of heat known that the AH comb = -891 mol, CH4 =[16 g/mol]

Yes she is correct. the answer is completely correct

60.0 - there are 3 significant numbers

We can't until we know what we are working with or on

Electron equations of the given changes are as follow,a)                              cu      →      cu⁺²  + 2e⁻b)                              mg⁺²  +  2 e⁻      →      mgc)                              cl⁻    →      cl  +  1e⁻d)                              s  +  2 e⁻    →      s²⁻e)                                o²⁻      →      o  +  2 e⁻f)                              p  +  3 e⁻    →      p³⁻g)                              fe²⁺    →      fe³⁺  + 1 e⁻h)                              s²⁻    →      s⁴⁺  +  6 e⁻