C+ax=dx

this one has all variables and no numbers

That's OK, but you have not said which variable you want to solve it for.

To solve for 'x':

c + ax = dx

Subtract c from each side: ax = dx - c

Subtract dx from each side: ax - dx = - c

Factor the left side: x (a - d) = - c

Divide each side by (a - d) : x = - c / (a - d) or x = c / (d - a).

To solve for 'c':

c + ax = dx

Subtract ax from each side and factor: c = x (d - a)

To solve for 'd':

c + ax = dx

Divide each side by 'x': d = c/x + a.

To solve for 'a':

c + ax = dx

Subtract 'c' from each side: ax = dx - c

Divide each side by 'x': a = d - c/x.

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