Finding exponential curve and r-squared using polyfit

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I have been trying to figure out how to find the exponential regression and r-squared given a set of x and y values as t and P. The equation I am given is P = a*e^(-b/t) This is my code

temp = [10:10:200];
pres = [1.227, 2.337, 4.241, 7.375, 12.340, 19.920,31.160,47.360, 70.110,101.330,143.270,198.540,270.130,361.380,476.000,618.060,792.020,1002.700,1255.100,1554.900]
tnew = 1.0 ./ (273.15+temp)
logp = log(pres)
AB = polyfit(tnew, logp, 1)
a = exp(AB(1))
b = -AB(2)
curve = a * exp(b./tnew)
r2 = 1 - sum(((a-b)./tnew-logp).^2)/ sum(( logp- mean(logp)).^2)