Question 292941
When does S(t)=15?
{{{t^2-2t=15}}}
{{{t^2-2t-15=0}}}
{{{(t-5)(t+3)=0}}}
Two solutions:
{{{t-5=0}}}
{{{t=5}}}
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{{{t+3=0}}}
{{{t=-3}}}
Since t represents time, a time of -3 seconds doesn't make sense for this problem. 
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{{{highlight(t=5)}}}