P=3L-3W
Move -3W to the other side. Sign changes to +3W
P+3W=3L-3W+3W
P+3W=3L
Divide both sides by 3 to get L by itself
P/3+3W/3=3L/3
Cross out 3 and 3, divide by 3 and then becomes 1*1*W=W
Cross out 3 and 3, divide by 3 and then becomes 1*1*L=L
P/3+W=L
Answer:
L=P/3+W
