The numbers given in the problem above are part of an arithmetic sequence with first and sixth terms equal to -21 and -36, respectively. Firstly, calculate for the common difference (d).
d = (-36 - -21) / (6 - 1) = -3
The arithmetic mean is calculated by adding -3 to the term prior to it.
a2 = -21 + -3 = -24 a3 = -24 + -3 = -27
a4 = -27 + -3 = -30 a5 = -30 + -3 = -33
Thus the four arithmetic means are -24, -27, -30, and -33.