6) twenty-one substracted from a number is -8
The equation for this statement is:

we substracted 21 from a number x.
Solving the equation:
add 21 to both sides of the equation:

The number is 13
7) A number increased by 91 is 37.
The equation for this statement:

To solve this equation, we substract 91 from both sides of the equation:

The number is -54