Well we know that dx/dt is equal to x'(t) or if we treat differentials kind of like numbers, and you can most of the time, this is not very rigorous, but you that that dx is equal to x'(t) dt and we know that dy of t or dy/dt, we know this is the same thing at the derivative of y with respect to t; y'(t) multiply both sides by dt you get dy = y'(t) dt. 