SAT Question 6ba39da5: Systems of Linear Equations answer and explanation with Desmos solution

Subtracting the second equation from the first equation in the given system of equations yields $\left(2a-2a\right)+\left(8b-4b\right)=198-98$, which is equivalent to $0+4b=100$, or $4b=100$. Dividing each side of this equation by $4$ yields $b=25$.