Hello!
On one side, the length is 6 mm on the small triangle, and on the big one, it's 10. The ratio is 6:10
You can make a proportion
[tex] \frac{6}{10} = \frac{12}{x} [/tex]
Cross multiply
10 × 12 = 120
Divide by 6 to get x
120 ÷ 6 = 20
[tex]\framebox{x = 20 mm}[/tex]
