+26400 (5a)2x 21a=4200
$$\begin{array}{rcl}
(5a)^2\cdot 21\cdot a &=& 4200 \\
(5a)^2\cdot 21\cdot a &=& 2\cdot 21 \cdot 100\\
(5a)^2 \cdot a &=& 2 \cdot 100 \quad | \quad \sqrt{}\\\\
5a \cdot a^{\frac{1}{2}} &=& 2^{\frac{1}{2} }*10 \quad | \quad :5 \\
a^1 \cdot a^{\frac{1}{2}} &=& 2^{\frac{1}{2} }*2^1 \\
a^{\frac{3}{2}} &=& 2^{\frac{3}{2} } \quad | \quad ()^{\frac{2}{3}}\\
a &=& 2
\end{array}$$
+130516 (5a)^2 * 21a = 4200
25a^2 * 21a = 4200
525 a^3 = 4200 divide by 525 on both sides
a^3 = 4200/ 525 = 8 take the cube root of both sides
a = 2
+26400 (5a)2x 21a=4200
$$\begin{array}{rcl}
(5a)^2\cdot 21\cdot a &=& 4200 \\
(5a)^2\cdot 21\cdot a &=& 2\cdot 21 \cdot 100\\
(5a)^2 \cdot a &=& 2 \cdot 100 \quad | \quad \sqrt{}\\\\
5a \cdot a^{\frac{1}{2}} &=& 2^{\frac{1}{2} }*10 \quad | \quad :5 \\
a^1 \cdot a^{\frac{1}{2}} &=& 2^{\frac{1}{2} }*2^1 \\
a^{\frac{3}{2}} &=& 2^{\frac{3}{2} } \quad | \quad ()^{\frac{2}{3}}\\
a &=& 2
\end{array}$$
