I'm currently readingsix not so easy pieces. It tells me in previous chapters we have seen that the laws of mechanics can be summarized by a set of three equations:

m(d^(2)x/dt^(2))= Fx but the x is a little subscript

The other two equations are the same but with a y substituted for x in one and a Z in the other.

Can anyone explain to me the variables because I have never seen this equation before and the book assumes I have. The funny part is the "previous chapters" part because this is located in the first chapter.