Question 1000137
This is telling you that {{{ y = 7 }}}
{{{ 7 = 15*( -cos( 6.28t/2.8 ) + 1 ) }}}
{{{ 7/15 =  -cos( 6.28t/2.8 ) + 1 }}}
{{{ 7/15 - 15/15 = -cos( 6.28t/2.8 ) }}}
{{{ 8/15 = cos( 6.28t/2.8 ) }}}
{{{ 8/15 = .5333 }}}
Let {{{ 6.28t/2.8 = theta }}}
{{{ .5333 = cos( theta ) }}}
Using the inverse cos function
and setting for radians 
on my calculator:
{{{ theta = 1.00826 }}}
{{{ 6.28t/2.8 = 1.00826 }}}
{{{ 6.28t = 2.8231 }}}
{{{ t = .4495 }}} sec
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The cos function ( all cos functions ) begins at {{{ 1 }}}, then 
goes to {{{ 0 }}} at {{{ theta = pi/2 }}}
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My value, {{{ theta = 1.00826 }}}, is less than {{{ pi/2 }}}, so
it is on the way down from zero inches from hand to
{{{ 15 }}} inches from hand when cos function is zero.  
When the cos is {{{ -1 }}},
yo-yo is {{{ 30 }}} inches from hand.
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To figure out all the times that yo-yo is {{{ 7 }}} in from hand
and on the way down, you must add {{{ 2*pi }}} like this:
{{{ 7 = 15*( -cos( 6.28t/2.8 + 2*pi ) + 1 ) }}}
Then you must find {{{ t }}} for {{{ 2*pi }}}, {{{ 4*pi }}}, {{{ 6*pi }}}, etc.
You will be on the way down each of those times and
{{{ 7 }}} in from hand
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Hope this helps