Question 254848
{{{3x + y = a}}}
{{{4x-y = b}}}
Solve for x in eq. 1 and substitute into eq. 2,
{{{3x + y = a}}}
{{{3x  = a-y}}}
{{{x=(1/3)(a-y)}}}
{{{4x=(4/3)(a-y)}}}
Now substitute,
{{{4x-y = b}}}
{{{(4/3)(a-y)-y = b}}}
{{{4(a-y)-3y = 3b}}}
{{{4a-4y-3y=3b}}}
{{{-7y=3b-4a}}}
{{{y=(1/7)(4a-3b)}}}