Look at the formula for the area of a circle below. . a = 3.14(pi)r2 . which of the following equations can be used to solve for the radius (r)?

You did not include the equations that you want to assess whether they can be used to solve for the radius (r). Likely, the equation of the circumference, C = 2*Pi*r is included, if so => r = C / (2*Pi). If you round Pi to 3.14, the equation may be written r = C / 6.28.

(x+a)(x+b)= x²+(a+b)x+ab

This is an identity of mathematics! not an equation.