In step 2, Carianne added 4 to the right side and subtracted 4 on the left side, which is completely changing the equation. By definition, your zero pair must have a difference of zero. She need to add 4 to both sides to continue; when manipulating algebra you need to do the same thing to both sides of an equation, otherwise you're just changing the equation.
The first one is 17.7 after rounding off a2+b2=c2 b2=19^2-7^2 b=17.664 The second one is 108 Volume of a prism=base area x length =(1/2 x12.6x1.6) x 10.75 =108.36mm3