A mixture contains two chemicals, A and B, with masses mA and mB. The total mass of the mixture, m, remains constant. B is converted into A at a rate that

is inversely proportional to the mass of A and directly proportional to the square of the mass of B in the mixture at any time t.

Form a differential equation and
find a general expression relating the mass of A to the time t.

ive got dma/dt = mb^2/ma

is this right and if so is the next bit

mb = m - ma
so dma/dt = (m-ma)^2/ma

then i get stuck