The third is because the prove is x = fifteen and if a number is next to a letter, it means multiply, sorry I could only do one.
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
This is the answer of the equation
The answer is negative 149