I solved the first problem this way, but I don't know how to approach the second part.
a[n_] := a1 + (n - 1) d
b[n_] := b1*2^(n - 1)
eq1 = a[2] - b[2] == a[3] - b[3];
eq2 = a[3] - b[3] == b[4] - a[4];
sol = Solve[{eq1, eq2}, {d, a1}] // Simplify
How should I tackle the second question?
