Formula use: \left(a-b\right)^3=a^3-b^3-3.a.b.\left(a-b\right)

Explanation

Here, given expression is :
\left(5x-8\right)^3-\left(3x-8\right)^3-6x\left(5x-8\right)\left(3x-8\right)
Let,
\left(5x-8\right)=a
\left(3x-8\right)=b
Now,
\left(a-b\right)
\rightarrow\{\left(5x-8\right)-\left(3x-8\right)\}
\rightarrow\left(5x-8-3x+8\right)
\therefore 2x
Now, Simplify the expression :
\left(5x-8\right)^3-\left(3x-8\right)^3-6x\left(5x-8\right)\left(3x-8\right)
\rightarrow\left(5x-8\right)^3-\left(3x-8\right)^3-3.2x\left(5x-8\right)\left(3x-8\right)
\rightarrow\left(5x-8\right)^3-\left(3x-8\right)^3-3.\{\left(5x-8\right)-\left(3x-8\right)\}.\left(5x-8\right)\left(3x-8\right)
\rightarrow a^3-b^3-3.a.b.\left(a-b\right)
\rightarrow\left(a-b\right)^3
\rightarrow\left(2x\right)^3
\therefore 8x^3
Ans: 8x^3\ .