How do i find the mistake the student did?

Step-by-step explanation:

What he did at the end of the given equations is solve for x in x + 8y= 21

x = 21 - 8y                Substitute that result in the top equation.

7(21 - 8y) + 5y = 14  is the correct step To continue Remove the brackets

147 - 56y + 5y = 14   Combine

147 - 51y = 14             Add 51y to both sides.

147 = 51y + 14            Subtract 14 from both sides.

133 = 51y                   divide by 51

y = 2.61 rounded.

The incorrect step is underlined and italicized.

x=7

Step-by-step explanation:

Perpendicular to the x axis means vertical, so every x coordinate will be the same, i.e., 7 because (7,2) is one of them. Hence the equation x=7.