2 7*a 2 a + --- - a 49
a^2 + (7*a)/49 - a^2 при a = 3
a^2 + (7*a)/49 - a^2
(3)^2 + (7*(3))/49 - (3)^2
3^2 + (7*3)/49 - 3^2
3/7
a - 7
0.142857142857143*a
7*a --- 49