Question 993724
Your teacher is right that:
Ave speed = ( total distance ) / ( total time )
Right away I see:
Total time =  {{{ 1.5 + 2 = 3.5 }}} hrs
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Did you say {{{ 750 }}} mi when you meant {{{ 750 }}} mi/hr ?
If so, 
{{{ 750*1.5 = 1125 }}}
{{{ 750 *2 = 1500 }}}
{{{ 1125 + 1500 = 2625 }}}
{{{ 2625/3.5 = 750 }}} mi/hr