There are a number of formulas that use two or more variables.
A=lxw The area of a rectangle is equal to the product of length and width.
`PV=nRT` is the ideal gas law where P is pressure, V is volume, n is the number of moles (a unit for measuring the number of atoms), R is the universal gas constant (this is not a variable), and T is temperature.
d=rxt distance = rate times time
`R=C/(C C)` the retail price = the cost divided by the cost complement
`F=G(m_1m_2)/d^2` The force of gravitational attraction F = the product of the gravitational constant and the masses of the two bodies divided by the square of the distance.