if x = √ 1 - t^2 , transform the differential equation

x (1 - x^2) d^2y/dx^2 + (1 - 3x^2) dy/dx - xy = 0

the answer to this problem is below:-

t (1 - t^2) d^2y/dt^2 + (1 – 3t^2) dy/dt - ty = 0

question is that how come the answer is similar to the question.

i dont think i just need to replace the x by t..

im confused..is something wrong with the answer??