Question 1059553
Perimeter of  a rectangle is  width+length+width+length (imagine walking around the rectangle, you will see the width twice & length twice), or more simply:  Perimeter = 2(width)+ 2(length) = 2(width+length).

Length is given as x-3
Width is given as 5x+4

We were given perimeter = 74cm:

  {{{ 2 ((x-3)+(5x+4)) = 74 }}}
  {{{ 2x-6+10x+8 = 74 }}}
  {{{ 12x+2 = 74 }}}
  {{{ 12x-72 = 0 }}}
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Answer:
  {{{ 12x-72 = 0 }}} 
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We could solve for x above (x=6), but it is not clear what x represents.
Let's check the answer using x=6:  length=6-3 = 3,  width = 5*6+4 = 34
      2*(3+34) = 2*(37) = 74    so at least we have the correct relationship.
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The question was to provide an equation for the perimeter.  Technically, the answer could've been given as   P(x) = 2[(x-3) +(5x+4)] ... simplified to  P(x) = 12x+2  as that is actually more of an expression of the perimeter as a function of x.   In this case, the information that the perimeter=74 is superfluous.