[tex]ax = 4\qquad |:a\\\\x= \frac4a[/tex]
[tex]P=2c+a\qquad |-2c\\\\P-2c=a\\\\a=P-2c\\\\\\P=2c+a\qquad |-a\\\\P-a=2c\qquad|:2\\\\\frac{P-a}2=c\\\\c=\frac{P-a}2[/tex]
[tex]\frac am=Z\qquad|\cdot m\\\\a=mZ\qquad|:Z\\\\\frac aZ=m\\\\m=\frac aZ\\\\\\\frac am=Z\qquad|\cdot m\\\\a=mZ[/tex]
