Which value of a makes the equation 13 = 40 − 3a true?

Which formula finds the probability that a point on the grid below will be in the blue area? p(blue) = total number of squares number of blue squares number of blue squares p(blue) = total number of squares number of blue squares p(blue) = number of white squares

Ineed in figuring out this problem.

Solve the equation by completing the square. x^2+2x=17