What is a given that : 3a/7 = -2/5

We'll use the cross multiplying to determine the value of
a: the products of means is equal to the product of
extremes.

7*(-2) = 5*3a

-14 =
15*a

Now, w'ell use symmetrical
property:

15a = -14

We'll
isolate a to the left side, dividing both sides by 15:

a =
-14/15.

Therefore, the requested value of the
unknown a is a = -14/15.