Hi there!
a = lw + 0.5bh
Let's start by flipping the equation
0.5bh + lw = a
Then add -lw to both sides
0.5bh + lw - lw = a - lw
0.5bh = -lw + a
Jesus Christ, we want to solve for b, but it is stuck between the other numbers.
So, to find the value for b, we need to divide both sides by 0.5h
0.5bh/0.5h = (-lw + a)/0.5h
b = (-lw + a)/0.5h
I hope this helps and pls let me know if you don't understand some steps :)
Best of luck!