Let $f(x)=(x-a)^3$. If the sum of all $x$ satisfying $f(x)=f(x-a)$ is $42$, find $a$.
1. Let \( f(x) = (x-a)^3 \). We need to find the value of \( a \) such that the sum of all \( x \) satisfying \( f(x) = f(x-a) \) is 42.
2. Define \( g(x) = f(x) - f(x-a) \). Notice that if \( x \) satisfies \( f(x) = f(x-a) \), then \( x \) is also a root of \( g(x) \).
3. Calculate \( g(x) \):
   \[
   g(x) = (x-a)^3 - (x-2a)^3
   \]
4. Use the difference of cubes formula:
   \[
   g(x) = (x-a)^3 - (x-2a)^3 = [(x-a) - (x-2a)] \left[(x-a)^2 + (x-a)(x-2a) + (x-2a)^2\right]
   \]
5. Simplify the expression:
   \[
   (x-a) - (x-2a) = a
   \]
   \[
   (x-a)^2 + (x-a)(x-2a) + (x-2a)^2 = x^2 - 2ax + a^2 + x^2 - 3ax + 2a^2 + x^2 - 4ax + 4a^2
   \]
   \[
   = 3x^2 - 9ax + 7a^2
   \]
6. Therefore:
   \[
   g(x) = a(3x^2 - 9ax + 7a^2)
   \]
   \[
   = 3ax^2 - 9a^2x + 7a^3
   \]
7. By Vieta's formulas, the sum of the roots of the polynomial \( 3ax^2 - 9a^2x + 7a^3 = 0 \) is given by:
   \[
   \frac{-(-9a^2)}{3a} = \frac{9a^2}{3a} = 3a
   \]
8. We are given that the sum of all \( x \) satisfying \( f(x) = f(x-a) \) is 42. Therefore:
   \[
   3a = 42
   \]
9. Solving for \( a \):
   \[
   a = \frac{42}{3} = 14
   \]

The final answer is \( \boxed{14} \).