[GAP Forum] Question regarding quotient group functions

[Daniel Grayham]

Hey guys, 
I have only recently started using GAP in my post graduate studies and I have come across a bit of difficulty.
This is a general example of the kind of code I have been working with, 
I generate my group by forming a free group and quotienting out the necessary relations, 
>From there I have been able to do almost everything, however when I try to form subgroups, say
Subgroup(G, [a^2]); 
where a is one of the generators of the group and say, a^2 has order 2 for example (generates subgroup isomorphic to C2) 
I get the error:
Error, families of <gens> and <M> are different
I
 have have double checked the generators of the group using 
GeneratorsOfGroup(G) and have also tried, for example if the group is 
only generated by a, 
Subgroup(G, [a]);

But I get the same error. 
I
 have gone through a lot examples where people use permutation groups 
and they work find for me, I only have trouble when I'm working with 
groups in this representation. 
The main reason
 I began trying to generate the subgroups was because I haven't been 
able to quotient out elements of the group, or use the FactorGroup(G,..)
 function. 
For example: 
G/[a^2]; Error, no method found! For debugging hints type ?Recovery from NoMethodFoundError, no 1st choice method found for `QUO' on 2 arguments
FactorGroup(G,
 Subgroup(G, [a^2]));   (ive tried Subgroup(..) in the position of a^2 
however the subgroup function gives a different error)Error, <N> must be a normal subgroup of <G>

Similarly with cosets, I haven't been able to find cosets in terms of elements, 
RightCosets(G, [a^2]); gives an error however:
RightCosets(G,Centre(G)); Works perfectly, 
Is there a crutial part of GAP code I am missing? 
Most importantly how can I find the quotient groups such as G/[a^2] for example? 
Thanks in advance, 
Grayham,