[tex]a) \\\\P=t-3rt+2r /+3rt-2r\\P+3rt-2r=t /-P\\3rt-2r=t-P\\r(3t-2)=t-P /:3t-2\\r=\frac{t-P}{3t-2}[/tex]
[tex]b)\\F=\frac{4y-z}{z} /*z\\Fz=4y-z /+z\\Fz+z=4y\\z(F+1)=4y /:F+1\\z=\frac{4y}{F+1}[/tex]
