Warm a solid and it grows — every metre of it, by a tiny fixed fraction per degree. The machine is $\Delta L = \alpha L_0 \Delta T$ (read aloud: delta-L equals alpha L-nought delta-T). $\Delta L$ is the growth in length — what you are solving for: $\alpha$ is alpha, the material's expansion coefficient per °C, $L_0$ is said “L-nought”, the length before the warmth, and $\Delta T$ is the temperature rise in C°. For steel, $\alpha = 12 \times 10^{-6}\ /^{\circ}\mathrm{C}$: twelve millionths of its length, per degree.

Twelve millionths sounds like nothing — until $L_0$ is a hundred-metre bridge deck and $\Delta T$ is a Canadian summer. A tiny fraction of a very long thing is real millimetres, which is why decks ride on expansion joints and rails are laid with gaps. Mind the units on the way out: the machine answers in metres, and the question usually wants millimetres — cross the ×1000 bridge last.