Compute the double summation:

∑ from i=0 to 2 ∑ from j=0 to 3 i²j³

Solve the following in terms of x [here |x| < 1]:

a) x³ - 2x² + x
b) x³ - x² + x
c) x⁴ - x³ + x²
d) x⁴ - 2x³ + x²