Can yall help me plz im deleting this account soon but i need the answer to this

Two customers went to a post office. first customer paid $12 for 14 and 5 envelopes the second paid 24.80 for 10 postcards and 15 envelopes. what is cost of each envelope?

[tex]5 \times1 \frac{12}{13} [/tex] me

Which statement best describes how to determine whether f(x) = x squared- x +8 is an even function ?