Area of whole triangle : whole rectangle
= 3 : 5

Difference = 5 units - 3 units = 2 units

Area of whole rectangle also
= shaded rectangle + unshaded rectangle
= 35cm² + 4 units

Area of whole triangle also
= shaded triangle + unshaded triangle
= 35cm² + 1 unit

Difference = 3 units

Since difference is the same either way we calculate it, we have to make the units the same.

3 units x 2 = 6 units
2 units x 2 = 6 units

So ratio of Area of whole triangle : whole rectangle = 3 x 3 : 5 x 3 = 9 : 15

So ratio of Area of shaded triangle : shaded rectangle = 1 x 2 : 4 x 2 = 2 : 8

Shaded area = 15 units - 8 units = 7 units
Shaded area also = 9 units - 1 unit = 7 units

So 7 units = 35cm²

1 unit = 35cm² ÷ 7 = 5cm²

Area of rectangle = 15 units = 15 x 5cm²
= 75cm²

Length of rectangle = 75cm² ÷ 6cm
= 12.5cm