Solving quadratic equations

I swear every time I get away from basic Algebra for a while I forget everything. Boy do I feel dense!

I have the following formula.

L = P * (P - 1)

But I want the formula so I can plug in L and get a value for P.

I kind of did it brute force and the best I could figure is that P = SQRT(L), then round up the result. But somehow I don't think that is exactly right.

Could someone please help me here? Please just don't give the answer show me how to factor it.

I think it is:

L= P^2 - P

but that is were I get stuck. Not sure how to proceed from there.

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