The parabola's line of symmetry is the x-axis.
Literal equation:
we know that
literal equation is equation of more one variable
for example: 
And we can solve such equations only if we are given two such equation
because it has two variables
One variable equation:
such equation has single variables
for example:

we don't need any other equation
we can solve for variables from single equation itself
because it has only one variable that is x
To find probability, you'd have to find the total number of marbles (5+3+6+6=20), and since there are 3 blue marbles, the probability of picking one is 3 out of 20, or 3/20
If that blue marble isn't replaced, there will then be 19 marbles, and 2 blue ones. So the probability of him picking another blue marble is 2/19
X3/4 is the answer for this question