Question 819718
You have to decide which variable to
plot on which axis.
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The independent variable goes on the x-axis
That would be the age.
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The dependent variable, pounds,  goes on the y-axis 
because the pounds <depends> on the age.
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Now you can say:
{{{ y = f(x) }}}, and you can use the general
point-slope formula to show the relation
{{{ ( 30 - 9 ) / ( 3 - 0 ) = ( y - 9 ) / ( x - 0 ) }}}
{{{ 21/3 = ( y-9 ) / x }}}
{{{ 7x = y - 9 }}}
{{{ y = 7x + 9 }}}
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How much will the child weigh at age 12?
{{{ y = 7*12 + 9 }}}
{{{ y = 84 + 9 }}}
{{{ y = 93 }}} pounds
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check the equation:
When child is born:
{{{ y = 7*0 + 9 }}}
{{{ y = 9 }}}
OK
When child is 3:
{{{ y = 7*3 + 9 }}}
{{{ y = 21 + 9 }}}
{{{ y = 30 }}}
OK