5) Lets use x for the unknown middle side. Since all the sides have to equal to perimeter, we can set everything equal 43.
(x-3)+x+(2x-2)=43
That would be your equation.
6) Knowing what we know from the previous problem, we can set the equation to 27.
(x-3)+x+4(x-3)=27 (we can plug in x-3 to represent the first side)
7) Since all the sides have to equal to perimeter, we can set everything equal 25.
a=2b-2
c=b+3
(2b-2)+b+(b+3)=25