Example 1 - mental math
Since the equation has infinitely many solutions, the coefficient of x in the left expression must be 9 and the constant in the right expression must be 6. Hence, k = 3 and c = 2.
Example 4 - mental math
It can be observed that the constant in left and right expression is same. For the equation to have no solution, 0.5a cannot be 10. Hence, a cannot be 20.