Question 1044562

Please help me solve this question in a understandable way for grade 10 curriculum!

Given that 3a−1/2a=7, determine the value of 27a^3−1/8a^3 without solving for a.

From the website, it gives that (3a−1/2a)(9a^2+3/2+1/4a^2)= 27a^3-1/8a^3, but how did they get the 3/2 from the middle? 

Answers will be much appreciated!
<pre>{{{3a - (1/2)a = 7}}}
{{{27a^3 - (1/8)a^3}}} =====> {{{(3a)^3 - ((1/2)a)^3}}}  
{{{(3a - (1/2)a)(9a^2 + 3a^2/2 + (1/4)a^2)}}} <====== This is what you should've seen
The above reduces to: {{{highlight_green(301a^2/4)}}}