Question 13187
The first thing to do is to assign variables for the sides of the triangle.  I am going to call them X, Y, and Z.  The first side, X, is 7cm shorter than twice the second side, Y.  Written as an equation, this is {{{X = Y - 7}}}.  Then, you know that the third side, Z, is 4cm longer than the first side.  WRite as an equation, this is {{{Z = X + 4}}}.  The perimeter, which is the sides added together, is 80.  So, {{{X + Y + Z = 80}}}.  

To solve this, you will need to figure out a way to get X + Y + Z = 80 to have only 1 variable in it. Since you know that Z = X + 4, you can plug X + 4 in for Z in that equation.  Now you have {{{X + Y + X + 4 = 80}}}.  For the first equation we wrote, solve for Y, which makes the equation {{{Y = X + 7}}}.  Now plug this in for Y in the longer equation, which changes it to {{{X + X + 7 + X + 4 = 80}}}.  To solve the equation, add like terms, which changes the equation to {{{3X + 11 = 80}}}.  Move the 11 to the right side of the equation, which is {{{3X = 69}}}.  Solve for X (X = 23).  To figure out Y and Z, simply plug 23 in for X in the original equations.  {{{23 = Y - 7}}}; Y=30.  {{{Z = 23 + 4}}}; Z=27.