56.6
step-by-step explanation:
the ratio of the perimeters is equal to the scale.
the ratio of the areas is equal to the square of the scale.
34 / 20 = k
a / 19.6 = k²
substituting:
a / 19.6 = (34 / 20)²
a = 19.6 (34 / 20)²
a ≈ 56.6
b