The width is half the length, so is
width = (1/2)*length
width = (1/2)*(3.2a + 0.18b) cm
width = (1.6a +0.09b) cm
The perimeter of the rectangle is twice the sum of length and width.
perimeter = 2*(length + width)
perimeter = 2*((3.2a +0.18b) cm + (1.6a +0.09b) cm)
perimeter = 2*(4.8a +0.27b) cm)
perimeter = (9.6a +0.54b) cm
Sasha did not get this answer, so apparently ...
her reasoning was not correct.