The breaking strength of three-stand Manila rope is given by the formula $$L=5000d(d+1)$$ where L lb. is the load needed to snap a rope d inches in diameter. What rope will just snap under a load of 60000 lb.? --Sawyer, Mathematician's Delight
We know that $$60000=5000d^2+5000d$$ How can I solve the equation to calculate d?