
Why?
We have the isosceles right triangle. The length of legs are 6. Use the Pythagorean theorem:

In general, the formula for calculating the hypotenuse in an isosceles right triangle is 
a - a leg, h - a hypotenuse
Depends
it can be a trinomial example
(x-3)(x-4)=x²-7x+12
it can be a binomial example
(x-3)(x+3)=x²-9
depends
We are trying to find the number that when added to 19, gives us less than 42. We can set up this simple inequality:
19 + x < 42
Now, subtract 19 from both sides:
x < 23
Our number can be anything less than 23.