# use pt() to find probability under the $t$-distribution pt(-3, df = 2) + (1 - pt(3, df = 2))
R calculates starting from the left of the graph. Therefore, to find the area below -3, you just do pt(-3, df=2) whereas you need to subtract the big area to the left of +3 (/starting from the right of the graph until 3) from 1, hence 1-pt(3,df=2)