Notice that he has a "-" followed by a ">" and you have the single character \[Rule].

There have been examples in the past where users have had "desktop published" characters that look almost identical to other characters, but that mean completely different things to Mathematica. But I am not claiming that is the cause of your problem.

Perhaps try typing

Plot[{x, 4x, 3x/2, (4/3)*(x - 10)}, {x, 0, 50}, PlotRange{0, 50}]

EXACTLY that way and then click to put the cursor between the e and the { and tap the - key and the > key and then Shift Enter and see if that works for you. (Any other characters and Mathematica may automatically translate the - and > into a single character.)

Copy and paste the code I displayed in my original reply; it should work just fine.

When you want to enter the abbreviation for Rule in an Input cell, type a hyphen (-) then a greater-than (>). The combination will automatically change to the correct symbol. Ditto if in the Input cell you type the longer form \[Rule].