Ok, so standard underwater swim.
First, before you submerge, you want to slightly hyperventilate. Do not over do this, as without practice, it can be dangerous.
The desire to breath is caused by CO2 build up, so take 3-4 very deep breaths and hold them for a second or to, then exhale completely. This will allow your body to reduce its CO2 load and store more Oxygen. If you over do this, you can reduce your CO2 load so much that you actually run out of Oxygen without the urge to breath and will pass out. So do not take more that 5 breaths.
The key is to burn as little energy as possible, you don't want to struggle or fight. You need to do an underwater frog kick, not a fin frog kick. Bring your legs toward you, then kick them out and back, when done your legs should be trailing behind you creating little drag. You have to use your arms. Bring you arms forward close to your body, under you face (looking face down) push your arms forward, at the same time as the frog kick, pull your arms to your side but not too far back, a little past the shoulders is fine, then bring your arms back in.
You need to glide between kicks and relax, but don't let all the momentum fade away before the next kick, as this requires you to use more energy on the next stroke.
Also, don't try for distance right away, get your strokes down. Once you get your strokes down, the 75' will be easy.
Don't worry, you can do this. It's simply a matter of a little practice, and remember, use your arms. You don't have fins, and you need your arms in this case.
Xanthro
