just to finish this method up, since Krizalid showed you another.
.............(1)
plug in in (1), we get:
plug in in (1), we get:
can we plug anything else in to eliminate B and C but not A? Nah, we will have to do A the hard way, but it will be easier considering we know B and C
expand (1):
group like powers of :
now the coefficients of each power of on the right must be the same as the coefficient of that same power on the left.
thus we have the system:
and we could have easily found each variable from this system to begin with (but i wanted to show you the two methods, you usually need only one). but now let's just find A. we can find A by plugging in the value we know for C in the equation