(10,50)(30,86)
slope = (86 - 50) / (30 - 10) = 36/20 = 1.8
y = mx + b
slope(m) = 1.8
(10,50)...x = 10 and y = 50
now sub and find b, the y int
50 = 1.8(10) + b
50 = 18 + b
50 - 18 = b
32 = b
ur equation is : y = 1.8x + 32
when fahrenheit is 20....y = 20
20 = 1.8x + 32
20 - 32 = 1.8x
-12 = 1.8x
-12/1.8 = x
-6.67 = x....celsius would be -6.67