"the perimeter of a swimming pool is 126 feet. the difference between the length (x) and width (y) of the pool is 39 feet. what are the dimensions of the pool? " i'm supposed to figure this out using substitution but i'm not sure how, and any at all is appreciated!

Ascientist has 50 grams of a radioactive element. the amount of radioactive element remaining after t days can be determined using the equation (1) after two days the scientist receives a second shipment of 50 grams of the same element. the equation used to represent the amount of shipment 2 remaining after t days is 10) - 50 which of the following is an equivalent form of the expression for the amount remaining in shipment 2? what’s the answer?