First, subtract color(red)(B) from each side of the equation to isolate the term containing P while keeping the equation balanced:
S - color(red)(B) = -color(red)(B) + B + (1/2)Pl
S - B = 0 + (1/2)Pl
S - B = (1/2)Pl
Next, multiply each side of the equation by color(red)(2) to eliminate the fraction while keeping the equation balanced:
color(red)(2)(S - B) = color(red)(2) xx (1/2)Pl
2(S - B) = cancel(color(red)(2)) xx (1/color(red)(cancel(color(black)(2))))Pl
2(S - B) = Pl
Now, divide each side of the equation by color(red)(l) to solve for P while keeping the equation balanced:
(2(S - B))/color(red)(l) = (Pl)/color(red)(l)
(2(S - B))/l = (Pcolor(red)(cancel(color(black)(l))))/cancel(color(red)(l))
(2(S - B))/l = P
P = (2(S - B))/l