Any two points in a flowing fluid are related by Bernoulli's equation:
ρgy1 + ½ρv12 + P1 = ρgy2 + ½ρv22 +P2
The trick in applying Bernoulli's equation properly is to throw away some of the terms right away.
Bring in the continuity equation to relate velocities:
A1v1 = A2v2