Question 842017
First try distributive property:
{{{i^5*i^2-3i*i^5}}}
{{{i^7-3i^6}}}
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From here, pay careful attention to the whole number powers of i.
You already know i^2=-1; and you understand that {{{(-1)(-1)=i^2*i^2=1}}} or {{{i^4=1}}}.
Showing the first few powers of i:
{{{i^0=what}}}
{{{i^1=i}}}
{{{i^2=-1}}}
{{{i^3=-i}}}------   (think how this makes sense.)
{{{i^4=1}}}
{{{i^5=i*i^4=i}}}
{{{i^6=i*i^5=-1}}}
{{{i^7=-i}}}
.
The pattern should tell you also that {{{i^0=1}}}.
Also notice how the values cycle.  They go in a cycle of i, -1, -i, 1; and this cycle repeats.


Back to the unfinished expression from your given one,
{{{i^7-3i^6}}}
{{{-i-3(-1)}}}
{{{highlight(-i+3)}}}  or  {{{highlight(3-i)}}}.