adelante is right in the sense that the distinction between the two is unlikely to be needed or helpful at A-level so I wouldn’t worry overmuch about it.
However, if you need to satisfy your teacher’s insistence that the distinction is important then the basis of Feminist-Marxism is an attempt to use the tools of Marxist analysis (class conflict, exploitation, domination, subordination etc.) as part of an explicit analysis of gender relationships. In Marxist-Feminism this tends to get a little lost in the sense that gender relationships tend to be seen as secondary / subordinate to class relationships.
In addition, Feminist-Marxism wants to focus on / bring further into the equation various forms of “non-productive” labour (both goods and services) performed mainly by women – through things like pregnancy, child-rearing, housework, subsistence work and so forth. These are usually subsumed into the general Marxist-Feminist “productive labour / exploitation” argument and, in such cases, get a little lost in the overall debate about the nature of “productive labour” (i.e. labour historically performed by men for money under capitalism). At A-level you often see this argument reduced to a “functional for men” analysis – capitalists exploit (male) employees who are then “forced” to exploit female partners within the family.
In basic terms, therefore, Feminist-Marxists place a stronger focus on gender relationships within capitalism, seeing the former as exploitative in their own right as a sub-division of capitalist forms of exploitation.