The First mistake she made is simplifying 4(x+5) - 3x to be 4x+5-3x.
The mistake here is that she did not expand the 4(x+5) correctly. She forgot to multiply 4 by 5 to correctly expand the bracket. The correct expansion of "4(x+5)" is "4x + 20" and not "4x + 5"
The second mistake is that she simplified 4x+5-3x to be equal to 6. This is wrong.
"4x+5-3x" gives "x + 5" and not 6.
The correct simplification of the problem is as follows:
4(x+5) - 3x = 4x + 20 -3x
Collect like terms
=4x - 3x + 20
=x + 20
The correct answer should be x + 20