Describe how to find the number of seats on the middle and lower levels of the stadium when solving for the variable only gives the number of seats on the upper level.

Use the value for the variable to solve for the other unknowns.  Substitute the value for the variable into the expression for the number of seats on the middle level.  Substitute the value for the variable into the expression for the number of seats on the lower level.

in a stadium normally it has the same number of seats on the upper and lower

& has 3 less in the middle