For similar figures the ratio of areas is the square of the ratio of linear dimensions.
We take "tripled in size" to mean that the linear dimensions of the larger rectangle are 3 times those of the smaller rectangle. That means the area of the larger rectangle will be 3² = 9 times the area of the smaller one.
Area = (3²)(20 cm²) = 180 cm² . . . . area after tripled in size
