Read the decal on the driver's door jamb.  It'll tell you exactly what the tire pressure should be or just read your owner's manual.

51 is the MAX it can handle. That's NOT what you keep the tires at. There should be a tag/sticker on the inside of the driver's door that says what the tire pressures should be. 

Here's an example of a sticker. Every vehicle is different so look at the one on your vehicle.

I agree with the guys. I can't say I haven't seen that on a tire before I have, I would just put them at 35. That's what I did with my kids car. It was fine after I did that he drives a 2008 Mazda 6. What kind of car do you drive? I'd personally go with what feels right when your driving you know your car best so going up to 35 wouldn't hurt it & probably feel better when your driving too.