Let the number of rectangles = R
Perimeter is 2(L) + 2(W)
The length remains the same when you add more triangles ( 6).
The width changes by multiplying the width by the number of rectangles.
Since the width of the rectangles are 1, multiply the number of rectangles R by 1 to get R.
Now replace W in the perimeter frmula with R:
Now the perimetr would be: P = 2L + 2R