Answer:
10.5 cm
Explanation:
The rectangle is organized like this:
Q----------------------R
50 4x+8
T-----------------------S
So you now know that QT and RS are sides similar to each other, so to finde the value of x you just put them in opposite sides of the equation:
4x+8=50
4x=50-8
4x=42
x=

x=10.5
By doing this you can observe that after evaluating the equation the value of X is 10.5