Describe and correct the error
9+2x3=11x3=33
19-6+12=19-18=1

Well,

In the transition between the first and second steps, whoever did this added before multiplying.  If we remember PEMDAS, we remember that Multiplication comes before Addition.

Whoever did this should have multiplied first before adding.

9 + 6
15

In the second equation, the error is that whoever did this did Addition before Subtraction.  You are supposed to do both Addition and Subtraction from left to right, whichever comes first.
19 - 6 + 12
13 + 12
25