To simplify the expression, we can first find a common denominator for the fractions inside the parentheses:

(а + 7)/(а - 7) - (а - 7)/(а + 7)

Multiplying the first fraction by (а + 7)/(а + 7) and the second fraction by (а - 7)/(а - 7), we get:

[(а + 7)^2 - (а - 7)^2]/[(а - 7)(а + 7)]

Expanding the numerator, we get:

[(а^2 + 14а + 49) - (а^2 - 14а + 49)]/[(а^2 - 49)]

Simplifying this further, we get:

(28а)/(а^2 - 49)

Now, we can simplify the denominator further by factoring it as the difference of squares:

(a^2 - 49) = (a + 7)(a - 7)

Therefore, the final simplified expression is:

28a/(a + 7)(a - 7)