Laplace transform of a linear input vs output line?

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I think my intuition of the Laplace transform and transfer functions is broken.

Suppose I have a linear function which relates two quantities r to x as such:

$$ r(x) = -100x + 25 $$

i.e. a straight line off center with nothing to do with time.

If I want a transfer function of the form $ \frac{R(s)}{X(s)} $, what should I do?

I'm expecting a constant (or DC gain), because this has nothing to do with time.

For example, $r(1) = -100 + 25 = -75$

If $\frac{R(s)}{X(s)} = c$, how could I get a value of 75 if I input a step function $ X(s) = \frac{1}{s}$?