The derivative you calculated in question 1 is an expression of the transistor's transconductance gₘ as a function of Vɢs. Calculate gₘ at Vɢs = 2.6 V and Vɢs =2.8 V in mA/V (or mS ).
Vₜₙ = −(p(2)/ 2p(1)) = −0.4850/2×0.1064 = 2.279 V