You know that BC is congruent to x so you need to solve for x using the ratio:
[tex]\frac{12}{24}=\frac{x}{BC}[/tex]
So then we need to find BC.
We know:
[tex]A=\frac{base \times height}{2}[/tex]
[tex]28=\frac{BC \times 7}{2}[/tex]
Therefore BC =8
Then:
[tex]\frac{12}{24}=\frac{x}{8}[/tex]
[tex]x=8 \times \frac{12}{24} = 4[/tex]
