Plz I need the answer very fast

Monica build a remote control toy airplane for science project to test the plane she launched it from the top of a building the plane travel a horizontal distance of 50 feet before landing on the ground a quadratic function which models the height of the plane in feet relative to the ground at a horizontal distance of x feet from the building is shown

Evaluate the expression for the given value of the variable 7a - 4a for a =8

Valentina is subtracting from . she finds the lcd to be 15y2. what is valentina's next step?