a)
[tex]F = \frac{mv^{2}}{2g} \ \ /\cdot2g\\\\mv^{2} = 2gF \ \ /:v^{2}\\\\\underline{m = \frac{2gF}{v^{2}}}[/tex]
b)
[tex]\frac{1}{f} = \frac{1}{x}+\frac{1}{y}\\\\\frac{1}{x} = \frac{1}{f}-\frac{1}{y}\\\\\frac{1}{x} = \frac{y-f}{fy}\\\\\underline{x = \frac{fy}{y-f}}[/tex]
