Step-by-step explanation:
[tex] \frac{6 {x}^{3} }{15 {x}^{4} - 60x ^{3} } \\ \\ = \frac{6 {x}^{3} }{3 {x}^{3} (5 {x}- 20 )} \\ \\ = \frac{2 }{5 ({x}- 4 )}, \: \: x \neq \: 4\\ \\ [/tex]
x = 4 makes the expression undefined.
Because at x = 4 denominator becomes zero, which makes the expression undefined.
