Question
$$x^2-\frac{21}{25}x = 0$$
Answer
$$ \begin{matrix}x_1 = 0 & x_2 = \dfrac{ 21 }{ 25 } \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} x^2-\frac{21}{25}x &= 0&& \text{multiply ALL terms by } \color{blue}{ 25 }. \\[1 em]25x^2-25 \cdot \frac{21}{25}x &= 25\cdot0&& \text{cancel out the denominators} \\[1 em]25x^2-21x &= 0&& \\[1 em] \end{aligned} $$
In order to solve $ \color{blue}{ 25x^{2}-21x = 0 } $, first we need to factor our $ x $.
$$ 25x^{2}-21x = x \left( 25x-21 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The second root can be found by solving equation $ 25x-21 = 0$.
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