To solve the first equation -4.2 + (у - 5.8) = 2.5, we first need to simplify the equation:

-4.2 + у - 5.8 = 2.5
у - 10 = 2.5

Now, add 10 to both sides of the equation:
у = 2.5 + 10
у = 12.5

So, the solution to the first equation is у = 12.5.

Next, let's solve the second equation -3.8 + (х - 4.2) = 3.6:

-3.8 + х - 4.2 = 3.6
х - 8 = 3.6

Now, add 8 to both sides of the equation:
х = 3.6 + 8
х = 11.6

Therefore, the solution to the second equation is х = 11.6.