If I have a 90 in a class and I don't do a project worth 45 percent, what will be my new grade?

On all my courses, that would result in failing the course. Every piece of set work had to be done and had to achieve at least a pass mark or you could not pass the course.

If the project is optional but worth 45% of the course mark overall and if there are no more projects or assignments that contribute to the course mark, then I reckon you'd end up with 45% of 0 + 55% of 90 which would be 49.5. That's not really the sort of number that says "employ me" or "accept me into the next course".

Well, the maximum you could possibly get would be somewhat less than 55, which is a failing grade in most classes.

And if you don't do well on the other assignments after this project, you'd get even less.

I bet you're flunking math, too.

Assuming that that 90% is the other 55%, you'll have 49.5%.

This isn't math, I guess, or you'd know the answer. If the 90% is the rest of the course, you've earned 90% of a total 55 marks. Do the math on that. Or just type 90% of 55 into Google, which acts as a calculator when you give it a formula. You'd fail the course.

You may fail.