Question

What did I do wrong in the highlighted parts

Answer 1

you did
D=(x2-x1)²+(y2-y1)²
it should be
D=
`√((x_2-x_1)^2+(y_2-y_1)^2)`

and BC, you did (-4)²=-16, it should be equal to 4

if you do the math which is evaluate

A=(5,3)
B=(1,5)
C=(3,0)

AB=
`√((1-5)^2+(5-3)^2)= 2√(5)`
BC=
`√((3-1)^2+(0-5)^2)= √(29)`
CA=
`√((5-3)^2+(3-0)^2)= √(13)`

add them to get total