Algebra

$3a + 4b = 5a - 6b + 24 - 3a$ combine like terms

$3a+4b=2a-6b+24$ add 6b to both sides

$3a+10b=2a+24$ subtract 2a from both sides

$a+10b=24$ subtract 10b from both sides

$a=24-10b$

And there is the solution, in terms of b.