Cancelling out Unkown Variables

Is there a way to write an equation with an unknown variable? The solution could then be displayed as a product of the variable, and then use another formula to cancel out the unknown variable.

In the attached screenshot, the n at the beginning of the first equation is unknown. Therefore the solution of s(eff)/n = 140 mm can be rearranged to s(eff) = 140*n mm. This can be then be cancelled out in the second equation to solve for K.

Note - In the screenshot (which is from a design guide), the guide confusingly uses n to define three different variables. In my workbook they are defined in order as n, n(1) and n(2), with n on the LHS of the equation being the unknown.