In 2002, the price of mailing a letter was $0.34 for the first ounce or part of an ounce and $0.21 for each ounce or part of an ounce after the first ounce. how much did it cost to mail a four-ounce letter?

The equation would be .34 + .21(x -1) = C  where x is the number of ounces and C = the cost.  Let x = 4 to answer the question.
.34 + .21(4-1) = C   Do the work in the parentheses
.34 + .21(3) = C   multiply
.34 + .63 = C   Add
.97 = C
The cost to mail the letter is $0.97.