Question

teorema de tales 4/12 x/21

Answer 1

$\frac{4}{12}= \frac{x}{21} \\ \\ 12*x = 4*21 \\ \\ 12x = 84 \\ \\ x = \frac{84}{12} \\ \\ x = 7$

Answer 2

12x = 84
x = 84/12
x = 7

é só multiplicar cruzado.