A)
[tex]8x^{2}-3x = 0\\\\x(8x-3) = 0\\\\x = 0 \ \ \vee \ \ 8x-3 =0\\\\x = 0 \ \ \vee \ \ 8x = 3 \ \ /:8\\\\\underline{x = 0 \ \ \vee \ x = \frac{3}{8}}\\\\\boxed{x \in \{0,\frac{3}{8}\}}[/tex]
B)
[tex]7x^{2}=21 \ \ /:7\\\\x^{2} = 3\\\\x^{2}-3 = 0\\\\(x+\sqrt{3})(x-\sqrt{3}) = 0\\\\x+\sqrt{3} = 0 \ \ \vee \ \ x-\sqrt{3} = 0\\\\\underline{x = -\sqrt{3} \ \ \vee \ \ x = \sqrt{3}}\\\\\boxed{x \in \{-\sqrt{3}, \sqrt{3}\}}[/tex]
C)
[tex]x(x+5)=4x(x+5)\\\\x(x+5)-4x(x+5) = 0\\\\(x+5)(x-4x) = 0\\\\x+5 = 0 \ \ \vee \ \ -3x = 0\\\\\underline{x = -5 \ \ \vee \ \ x = 0}\\\\\boxed{x\in\{-5,0\}}[/tex]