We have the next relation
[tex]\frac{12}{15}=\frac{6.4}{x}[/tex]
then we need to clear x
[tex]x=\frac{6.4\cdot15}{12}=8[/tex]
the scale factor to have the bigger triangle from the smaller triangle is
[tex]\frac{5}{4}[/tex]
for example
[tex]undefined[/tex]
