We have 3 unkown terms x, y and a.
In the figure below angles marked in red are equal to each other and angles marked in blue are equal to each other.
Attachment:
Inkedlines_LI.jpg [ 821.23 KiB | Viewed 6743 times ]
So from there we can write:
\(x=ay\) and \(x+10=y\)
Also angle marked in yellow is 180 because it lies on a line
\(x+y+120=180\) or \(x+y+10=70\) or \(x+10+y=70\) or \(y+y=70\) (from the second equation above) or \(y=35\) and \(x=25\)
\(ay=x\) or \(a=\frac{x}{y}=\frac{25}{35}=\frac{5}{7}\).