Geometric optic, displacement formula f=(L^2-d^2)/4L

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Stupid question I am stuck at the displacement formula in geometric optic I am following the steps at schoolphysics. The steps are the same more or less. But I cant for my mind see the steps wich ends up at

u = [d – a]/2 and v = [d + a]/2

and everywhere I look they just state it driving me nuts. Can someone please explain how they get to the expression?

Thanks!