We need to leave m alone on the right hand side, so let's move everything else to the left hand side.
First of all, we can subtract b from both sides to get
[tex] y-b = mc [/tex]
Now, we can divide both sides by c to get
[tex] \dfrac{y-b}{c}=m [/tex]
And so we solved the expression for m, because we have written an expression in the form [tex] m=\ldots [/tex], where the right hand side doesn't depend on m.
