now, we're assuming she made the deposit at the child's birth, namely when year was 0, at the child's twenty-first birthday that'll be 21 years later.

2000, you just need to put three zeros at the end of the number.
90. Add the zero in front of the 9.
A = l*w
A = 4*12 = 48cm^2
The area of the rectangle is 48cm^2 (cm squared)