In a regression analysis involving 27 observations, the following estimated regression equation was developed.

y = 25.2 + 5.5x1For this estimated regression equation SST = 1550 and SSE = 520. 
a. At ± = .05, test whether x1 is significant. Suppose that variables x2 and x3 are added to the model and the following regression equation is obtained. 

y = 16.3 + 2.3x1 + 12.1x2 - 5.8x3For this estimated regression equation SST = 1550 and SSE = 100. 
b. Use an F test and a .05 level of significance to determine whether x2 and x3 contribute significantly to the model.