Question 106448
Let's call the pieces A and B, where B is the longer piece.
You know that 
1.{{{A+B=18}}}
and also that
2.{{{B=A+4}}}
You can substitute the expression for B from equation 2 into equation 1. 
1.{{{A+B=18}}}
{{{A+(A+4)=18}}}Substitute for B.
{{{2A+4=18}}}Simplify.
{{{2A+4-4=18-4}}}Additive inverse of 4.
{{{2A=14}}}Simplify.
{{{2A/2=14/2}}}Multiplicative inverse of 2.
{{{A=7}}}Final answer. 
If A=7 feet then you can go to equation 2 and,
2.{{{B=A+4}}}
{{{B=7+4}}}
{{{B=11}}}
The two pieces of wood are 7 feet and 11 feet long.