Найдите значение выражения 7a/3c-49a^2+9c^2/21ac+3c-49a/7a, при a=78,c=20

(7а/3с)–((49а2 + 9c2)/21ac)+((3c–49a)/7a) =
=(49a2/21aс)–((49а2 + 9c2)/21ac)+(3c(3c–49a)/21ac)=
=(49a2–49a2–9c2+9c2–147ac)/21ac=–147ac/21ac=–7