a)
[tex]a = \frac{b}{c} \ \ \ |\cdot c\\\\ac = b \ \ \ |:a\\\\\boxed{c = \frac{b}{a}}[/tex]
b)
[tex]s = \frac{at^{2}}{2} \ \ \ |\cdot2\\\\at^{2} = 2s \ \ \ |:t^{2}\\\\\boxed{a = \frac{2s}{t^{2}}}[/tex]
c)
[tex]z = a+3b\\\\3b = z-a \ \ \ |:3\\\\\boxed{b = \frac{z-a}{3}}[/tex]
d)
[tex]c(x-y) = z\\\\\boxed{z = c(x-y)}[/tex]
