Just say M=10 and A is the unknown you can solve it for F but when M and A are unknown how to you solve it. My friend and physics told me that it can be solves for the unknowns using systems of equations, but i never learnt that. For F=MA to solve for two unknown you need to use two other equation M=F/A and A=F/M its called systems equations i think.
With simultaneous equations and multiple unknowns, you need several pieces of information. Generally you need one equation for each unknown quantity. So if you have two unknowns you need two equations.
And you can't "cheat" by rearranging the first equation and using that as the second, because when you try to solve it you will end up getting an equation like or that is as useless as it is true.
For example the equation has infinitely many possible solutions for and , so you need more information to go off.
Suppose you are then given .
So now you have two equations:
We can rearrange the second equation, giving us and then we can substitute this back into the first equation, giving:
And from there it is possible to solve for . Once you know you can then solve for .
In some more common cases it may be better to add the equations together:
Or subtract them:
But ultimately what you're trying to do is to focus on one variable by creating an equation without all the others. Once you have that variable you can use it to find the other(s).