ca = ec
cpctc makes it so that corresponding parts of the triangle you proved to be equal are equal, the other options aren't part of the part of the triangles you proved to be equal.
at time x=0 we have 1000, at time x=1 we have 2000, at time x=2 we have 4000.
so that's an initial value of 1000 and a ratio of 2,
y = 1000 (2^x)
hope this !