To solve for a variable, you isolate that particular variable on one side of the equation and everything else on the opposite side.
So, we have the equation g = ca. And on the left-hand side is the variable a.
That is the variable we want to solve for.
Now let's think how we can isolate the variable a on its own.
Well, we can divide the ca term by c. The c's will cancel out and we will be left with just the variable a.
But remember. Anything we do on one side of the equation, we must do on the other side.
g / c = ca / c
g/c = a
So, a is equal to g/c.