Question 994534
plot your points, let time be {{{x}}} and temperature {{{y}}}

{{{xmin}}}|{{{y}}}º
{{{3min}}}|{{{93.5}}}º 
{{{5min}}}|{{{90}}}º 
{{{9min}}}|{{{84.8}}}º 
{{{18min}}}|{{{70.2}}}º 
{{{30min}}}|{{{54.4}}}º 
{{{37min}}}|{{{42.5}}}º 
{{{48min}}}|{{{24.8}}}º 
 


{{{drawing( 600, 600, -10, 60, -10, 100,
circle(3,93.5,.25),
circle(5,90,.25),
circle(9,84.4,.25),
circle(18,70.2,.25),
circle(30,54.4,.25),
circle(37,42.5,.25),
circle(48,24.8,.25),

locate(3,93.5,p(3,93.5)),
locate(5,90,p(3,90)),
locate(9,84.4,p(9,84.4)),
locate(18,70.2,p(18,70.2)),
locate(30,54.4,p(30,54.4)),
locate(37,42.5,p(37,42.5)),
locate(48,24.8,p(48,24.8)),

 graph( 600, 600, -10, 60, -10, 100, 0)) }}}


To calculate the slope, pick two points on the line, and calculate the slope as:

 {{{slope=(y[2]-y[1]) / (x[2]-x[1])  }}}(your slope should have a negative value).

The equation of the line is then 

{{{y = slope *x + b}}} where {{{b=y-intercept}}}

I would pick these two points on the line:

({{{9min}}},{{{84.8}}}) 
({{{48min}}},{{{24.8}}}) 

 {{{slope=(y[2]-y[1]) / (x[2]-x[1])  }}}

 {{{slope=(24.8-84.8) / (48-9)  }}}

{{{slope=-60 / 39  }}}

{{{slope=-20/13  }}}

{{{slope=-1.54  }}}

so, equation is {{{y = -1.54x + b}}}

use one point to find {{{b}}}

{{{84.8 = -1.54*9 + b}}}

{{{84.8 +1.54*9= b}}}

{{{84.8 +13.86= b}}}

{{{b=98.66}}}

{{{y = -1.54x + 98.66}}}


{{{drawing( 600, 600, -10, 60, -10, 100,
circle(3,93.5,.25),
circle(5,90,.25),
circle(9,84.4,.25),
circle(18,70.2,.25),
circle(30,54.4,.25),
circle(37,42.5,.25),
circle(48,24.8,.25),

locate(3,93.5,p(3,93.5)),
locate(5,90,p(3,90)),
locate(9,84.4,p(9,84.4)),
locate(18,70.2,p(18,70.2)),
locate(30,54.4,p(30,54.4)),
locate(37,42.5,p(37,42.5)),
locate(48,24.8,p(48,24.8)),

 graph( 600, 600, -10, 60, -10, 100, -1.54x + 98.66)) }}}