Answer
An equation: $F=\displaystyle \frac{9}{5}C+32,$
$70^{o}F\approx 21.1^{o}C$
Work Step by Step
We are given two $(C,F)$ points
$(0,32)$ and $(100,212)$.
Calculate slope: $m=\displaystyle \frac{212-32}{100-0}=\frac{9}{5}$
Apply the line equation $y-y_{1}=m(x-x_{1})$
F representing y, and C representing x,
$F-32=\displaystyle \frac{9}{5}(C-0)$
$F=\displaystyle \frac{9}{5}C+32$
When F=70,
$70=\displaystyle \frac{9}{5}C+32\quad... /-32$
$38=\displaystyle \frac{9}{5}C \quad... /\times\frac{5}{9}$
$C=\displaystyle \frac{5\cdot 38}{9}\approx 21.1$
