Question 27958
First you need to know a little bit of geometry vocabulary, namely that an isosceles triangle has two sides the same, called legs, and a base that is not similar. The perimeter is defined by the sum of the lengths of all sides, so for an isosceles triangle that would be base+leg1+leg2.
We'll call the length of the base "x" and the problem tells us that each leg is "4m longer than the base" so we'll write for each leg "4+x".
When we add them all up we get 
{{{perimeter=base+leg1+leg2}}} We're told the perimeter is 26.
{{{26=x+(4+x)+(4+x)}}}Combining x's and constants:
{{{26=3x+8}}}Subtract 8 from both sides:
{{{18=3x}}}Divide by 3:
{{{6=x}}}
So x, the base for this triangle, is 6 and the two legs are 6+4=10. So we have a triangle with sides 6, 10, and 10 with a total perimeter of 26.