To solve this equation, we need to simplify the left-hand side first.

(-5/14) × 21/25 - y = 3 3/4

First, let's calculate (-5/14) × 21/25:

(-5/14) × 21/25 = -105/350 = -3/10

Now, the equation becomes:

-3/10 - y = 3 3/4

Next, let's convert 3 3/4 to an improper fraction:

3 3/4 = 15/4

So our equation now looks like:

-3/10 - y = 15/4

To solve for y, we can add 3/10 to both sides of the equation:

-3/10 - y + 3/10 = 15/4 + 3/10
-y = 15/4 + 3/10
-y = 75/10 + 3/10
-y = 78/10
y = -78/10
y = -39/5

Therefore, the solution to the equation is y = -39/5.