How do you solve systems of equations by elimination

The area of a circle is equal to 1 dm². find the radius of the circle.

Atruck can be rented from company a $50 a day plus $0.60 per mile. company b charges $20 a day plus $0.80 per mile to rent the same truck. find the number of miles in a day at which the rental costs for company a and company b are the same

What is the value of the discriminant, b2 ? 4ac, for the quadratic equation 0 = x2 ? 4x + 5, and what does it mean about the number of real solutions the equation has?