Question 551097
<pre>

-2, ____, ____, ____, 12

If the common difference is d, then to get the next term, 
we just add d each time.  So to fill in the blanks:

-2, <u>-2+d</u>, <u>-2+2d</u>, <u>-2+3d</u>, -2+4d=12

Solve the last equation

-2+4d = 12
   4d = 14
    d = {{{14/4}}}
    d = {{{7/2}}}

 -2, -2+{{{7/2}}}, -2+2{{{(7/2)}}}, -2+3{{{(7/2)}}}, 12

 -2, {{{-4/2}}}+{{{7/2}}}, {{{-4/2}}}+{{{14/2}}}, -{{{-4/2}}}+{{{21/2}}}, 12

 -2, {{{3/2}}}, {{{10/2}}}, {{{17/2}}}, 12
 
 -2, {{{3/2}}}, 5, {{{17/2}}}, 12

So the three arithmetic means between them are {{{3/2}}}, 5, and {{{17/2}}} 

Edwin</pre>