Hey there!
Let's write our expression below.
5x+50
A proportional relationship can be written in the form y=kx, so let's rewrite our expression as an equation.
y=5x+50
A proportional relationship must pass through the origin. The y-intercept has to be 0. y=5x would be proportional, but we have y=5x+50, so therefore it is not proportional.
I hope this helps!