When a Steel Cookie Sheet Is Placed in an Oven $$\Delta _ { A } = A _ { 0 } \gamma \Delta T$$

Essay

When a steel cookie sheet is placed in an oven and heated up, it expands due to the
temperature change.The change in area of the cookie sheet is modeled using the following
equation: $$\Delta _ { A } = A _ { 0 } \gamma \Delta T$$
where
$$\begin{array} { l } \Delta _ { A } = \text { change in area in } \mathrm { mm } ^ { 2 } \\A _ { 0 } = \text { original area in } \mathrm { mm } ^ { 2 } \\\gamma = \text { area coefficient of thermal expansion } \\\Delta _ { T } = \text { change in temperature in } { } ^ { \circ } \mathrm { C }\end{array}$$
What is the appropriate unit for $\gamma$ if the preceding equation is to be homogeneous in units?