We need to compute the second derivative and look for those x values that make it 0 (aka, critical points)
y'' = 7(7x^2 - 1) e^{-3.5x^2}
So, x = ±√1/7 are two critical points. Concavity sign is determined by looking at the sign chart, which says concavity is positive for x<-√1/7 or x>√1/7; negative, otherwise