You can put this solution on YOUR website! This is not a linear equation, slope nor graph problem.
There are many equivalents not just "the" equivalent.
An equivalent is 6a+8b-(-12a-6b)
but I suspect that is not what you really meant to ask.
multiply everything in the parentheses by -1
because 3a + 4b - (-6a - 3b)
means 3a + 4b -1* (-6a - 3b)
Work out (-1)*-6a)+(-1*)*(-3b)
then add that to 3a+4b