Question 162646
It's the distributive property.
You are distributing the (-1) that is in front of the parentheses.
Here it is step by step. 
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{{{(8-(Y+4))=8-1(Y+4)}}}
{{{(8-(Y+4))=8+(-1)(Y+4)}}}
{{{(8-(Y+4))=8+((-1)(Y)+(-1)(4))}}}
{{{(8-(Y+4))=8+(-Y)+(-4)}}}
{{{(8-(Y+4))=8-Y-4}}}