Expand ${\left(5x\right)}^3$ so that no bracket is left.

Power of a product: every factor inside the bracket takes the power, so ${\left(5x\right)}^3=5^3\times x^3=125x^3$. The trap is applying the power only to the variable $(5x^3)$ or only to the numeral $(125x)$; $15x^3$ comes from multiplying $5\times 3$ instead of cubing 5.