Let's simplify or solve each expression you provided step by step.

### a) \(5y - 4x - 8 - (y + 2x - 3)\)

1. Distribute the negative sign:
\[
5y - 4x - 8 - y - 2x + 3
\]
2. Combine like terms:
- \(5y - y = 4y\)
- \(-4x - 2x = -6x\)
- \(-8 + 3 = -5\)

So, the simplified expression is:
\[
4y - 6x - 5
\]

### b) \((2x - y)(4x - 3y) - 20z^3y^2 : (-2x^2y)\)

1. First, multiply \( (2x - y)(4x - 3y) \):
\[
2x \cdot 4x + 2x \cdot (-3y) - y \cdot 4x - y \cdot (-3y) = 8x^2 - 6xy - 4xy + 3y^2
\]
Combine like terms:
\[
8x^2 - 10xy + 3y^2
\]

2. So, now we have:
\[
8x^2 - 10xy + 3y^2 - 20z^3y^2
\]
Combine \(3y^2 - 20z^3y^2\):
\[
8x^2 - 10xy + (3 - 20z^3)y^2
\]

3. Now, let's divide by \(-2x^2y\):
\[
\frac{8x^2 - 10xy + (3 - 20z^3)y^2}{-2x^2y}
\]
Break it into three separate fractions:
\[
\frac{8x^2}{-2x^2y} + \frac{-10xy}{-2x^2y} + \frac{(3 - 20z^3)y^2}{-2x^2y}
\]

Simplifying each term yields:
\[
-4/y + \frac{5}{x} + \frac{-(3 - 20z^3)y}{2x^2}
\]
Which can further be combined into:
\[
-\frac{4}{y} + \frac{5}{x} - \frac{(3 - 20z^3)}{2x^2}y
\]

### a) \(x^2 - 10x + 25 - y^2\)

1. We recognize that \(x^2 - 10x + 25\) can be rewritten as \((x - 5)^2\):
\[
(x - 5)^2 - y^2
\]
2. This expression is a difference of squares:
\[
(x - 5 - y)(x - 5 + y)
\]

### b) \(x^3 + y^3 - 3x - 3y\)

1. We can apply the sum of cubes factorization here:
\[
(x+y)(x^2 - xy + y^2) - 3(x + y)
\]
Factor out \((x+y)\):
\[
(x+y)(x^2 - xy + y^2 - 3)
\]

### c) \(x^3 + 2x^2y + xy^2 - 4x\)

1. We can factor out \(x\):
\[
x(x^2 + 2xy + y^2 - 4)
\]
2. The quadratic inside can be tricky. It doesn't factor nicely, but we have:
\[
x(x^2 + 2xy + y^2 - 4)
\]

Thus, we've simplified each expression as much as possible. If you need more details on any specific part or additional assistance, feel free to ask!