System equations by substitution i dont what to do i wasnt in class for a whole week so i dont know what i'm doing and i cant ask my teacher since i have subitute

Find the volume of the described solid. the solid lies between planes perpendicular to the x-axis at x = 0 and x=7. the cross sectionsperpendicular to the x-axis between these planes are squares whose bases run from the parabolay=-31x to the parabola y = 30/x. a) 441b) 147c) 864d) 882

Including 6% sales tax, an inn charges $135.68 per night. find the inns nightly cost

Ingredient c: 1/4 cup for 2/3 serving or ingredient d: 1/3 cup for 3/4 serving which unit rate is smaller