Let tram speed be $a$ km/h.

For both overtakes, the length (L) is the same: $L$ = (relative speed w.r.t. X) × 8 s = (relative speed w.r.t. Y) × 9 s

Relative speeds (in km/h): $(a - 3)$ and $(a - 4)$. The conversion to m/s cancels when equating: $8(a - 3) = 9(a - 4) \Rightarrow 8a - 24 = 9a - 36 \Rightarrow a = 12$ km/h

Now compute $L$ using X: Relative speed = $(12 - 3) = 9$ km/h = $9 \times (5/18) = 2.5$ m/s $L = 2.5 \times 8 = 20$ m