The problem was:

Suppose that the slope of a regression line is 6 and the standard error of the slope is 3. There are 81 observations. Is the slope significantly greater than zero? What is a 95 percent confidence interval for the slope?

The ratio of the slope to the standard error is 6/3 = 2.0, which is a t variable. You do tcdf(2.0, 100, 79) because the degrees of freedom is n-2 = 79. My guess is that it comes out close to .05, probably less, and is significant.

The 95 percent confidence interval with 79 degrees of freedom has a t* of about 1.99, so call it 2. Then the margin of error is t*(s) = 2(3) = 6. So the 95 percent confidence interval is b + or - m, and since b and m are both 6, the confidence interval is (0,12). This tells me that with a two-sided test the p-value would be .05, and with a one-sided test it would be .025, so it confirms that it is significantly greater than zero.