TeX source:

\begin{array}{lll} h \text { is } f \text { of } a & \text { We name the function } f ; \text { height is a function of age. } \\ h=f(a) & \text { We use parentheses to indicate the function input. } \\ f(a) & \text { We name the function } f ; \text { the expression is read as "} f \, \text {of } \, a". \end{array}