Step 1: simply both sides of the equation.
4 + 3a - 6 = 43
4 + 3a + (-6) = 43
(3a) + (4 + -6) = 43 (combine like terms)
3a + -2 = 43
3a - 2 = 43
Step 2: Add 2 to both sides.
3a - 2 + 2 = 43 + 2
3a = 45
Step 3: Divide both sides by 3.
3a/3 = 45/3
a = 15