Deriving displacement function from gravity constant

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Homework Question: The velocity (v) of an object is the rate of change of position (y(t)), and the acceleration is the rate of change of velocity. If the acceleration of an object falling vertically is constant and equal to -g, write a differential equation and initial conditions for the position of the object and solve it by integrating twice, if the object is stationary at a height of y meters at the starting time.

I ended up with y(t)=-1/2gt^2+kt+C (where k and c are arbitrary constants) y(0) = C = 0 y(t) = -1/2gt^2+kt

However, the solution just had y(t) = -1/2gt^2

Am I wrong because I kept two arbitrary constants in y(t)=-1/2gt^2+kt+C and didn't combine them to be y(t)=-1/2gt^2+ct ???

I have been getting confused with knowing what i can and cannot do with the constants after integrating. What if there are two? What if one is multiplied with the time function while the other isn't, can i still combine (like above)?

Thank you in advance!