Answer:
the length of the third side is 21
Explanation:
let P be the perimeter of the triangle
a the length of the first and second sides
b the length of the 3rd side
…………………………………………………
Then
P = a + a + b
= 2a + b
Where b = a + 6
Therefore
P = 2a + (a + 6)
= 3a + 6
We substitute the numbers we have into the equation:
51 = 3a + 6
⇔ 45 = 3a
⇔ a = 15
Since b = a + 6 then b = 15 + 6 = 21