A) The negative rate indicates the glacier is getting smaller ( it is melting).
B) Multiply the size by the rate of change then subtract:
58850 x 0.0004 = 23.54 miles.
58,850 - 23.54 = 58,826.54 square miles.
C) A decade is 10 years.
Multiply the rate of change by 10: 0.0004 * 10 = 0.004
(1 - 0.004)^d or (0.996)^d, where d is the number of decades.