<span><span>(<span><span>12</span><span>(<span>3+7</span>)</span></span>)</span><span>(3.5)</span></span><span>=<span><span><span>12</span><span>(<span>3+7</span>)</span></span><span>(3.5)</span></span></span><span>=<span><span><span>12</span><span>(<span>3+7</span>)</span></span><span>(3.5)</span></span></span><span>=<span><span>(<span><span>12</span><span>(10)</span></span>)</span><span>(3.5)</span></span></span><span>=<span><span>(5)</span><span>(3.5)</span></span></span><span>=17.5
</span>

Hope you could get an idea from here.
Doubt clarification - use comment section.
Multiple 6 with numbers inside paranthesis:
12x - 66 + 15 = 21
add 66 fhen substrack 15 from both sides :
12x = 72 divide both sides with 12:
x = 6
<u>Answer:</u>
4.5 foot is needed for a unit dozen bird house
<u>Explanation:</u>
Given Tina saw ¾ foot of wooden trim is being used for first 1/6 dozen bird houses
Therefore, we can write as,4
1/6 dozen = ¾ foot
For 1 dozen bird houses, we have to use the unitary method
1/6 dozen = ¾ foot
Therefore, 1 dozen or unit dozen = 
1 dozen = ¾ * 6 =
foot = 4.5 foot
Therefore, 4.5 foot is needed for a unit dozen bird house