I don't understand this subject

Answer:
7.3
Step-by-step explanation:
The two triangles are similar, meaning that they have the same angles but the triangles are in different sizes. To solve this, use proportions:
[tex]\frac{3}{5} =\frac{4.5}{x}[/tex]
Cross multiply:
[tex]3x=22.5\\x=7.3[/tex]