similar triangles

The sides are being multiplied by $5$, so $3x+8=4*5=20\implies 3x=12\implies\boxed{x=4}$.

$\frac{3x+8}{15} = \frac{4}{3}$

$\frac{3x+8}{15} = \frac{20}{15}$

3x + 8 = 20

3x = 12

$\boxed{x = 4}$