How can i write a system between 2 derivates with the ti-89?

I mean

d/dx [f(x,y)=a]

where

dy/dx=c

for example if f(x,y)=a is x^2+3 =y

then the solution is 2x=c => x=c/2

I wasn't able to find anything... i've tried to do something like

solve(d(x^2+3=y,x)|(d(y,x)=1),x) -->wrong value

d(x^2+3=y,x)|(d(y,x)=1)) ------->2x=0 wrong

Thanks in advance

Marco