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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