The pressure inside a hydrogen-filled container was 2.10 atm at 21 ∘
c. what would the pressure be if the container was heated to 98 ∘c ?

According to Gay's Lussac law, we know, at constant Volume, Pressure is directly proportional to temperature. 
So, P/T = constant 

P₁/t₁ = P₂/t₂
P₁t₂ = P₂t₁

Here, we have: P₁ = 2.10 atm
P₂ = ?
T₁ = 21+273 = 294 K
T₂ = 98+273 = 371 K

Substitute their values into the expression:
2.10 × 371 = 294 × P₂
P₂ = 779.1 / 294
P₂ = 2.65 atm

In short, your final answer would be 2.65 atm

Hope this helps!