Answer
The second line should equate the product of the means and the product of the extremes. 7x=12. Then, the third line would be x=$\frac{12}{7}$
Work Step by Step
We are given the proportion $\frac{7}{3}$ = $\frac{4}{x}$
We use the cross product $\frac{a}{b}$ = $\frac{c}{d}$, which equals ad=bc
So: 7x = 3(4); or 7x = 12
Divide both sides by 7 to get a correct answer of x = $\frac{12}{7}$
