Question

The profit P (in dollars) from selling x calculators is P =25x-(10x+250). How many calculators are sold when the profits are $425?

Answer 1

You probably have done something like this and are confused by the directions. 
Step One 
Write down the equation and the givens.
P = 425
x is what you are trying to isolate.
P = 25x - (10x + 250)

Step Two
Remove the brackets on the right.
P = 25x - 10x - 250 Notice all those minus signs. The one on the outside of the brackets gets distributed just as thought there was a (say) - 5

Step Three
Collect the like terms on the right.
P = 15x - 250 

Step Four
This really doesn't require a separate step, but I should have done it at the beginning. Put in P = 425
425 = 15x - 250

Step 5
Add 250 to both sides.
425 + 250 = 15x
675 = 15x 

Step Six
divide by 15
675 / 15 = x
45 = x

He has to sell 45 calculators.

Answer 2

To solve this all you need to do is plug 425$ in for P.

425 = 25x - (10x + 250)

*Distribute the negative sign*

425 = 25x - 10x - 250

*Add 250 on each side*

675 = 25x - 10x

*Combine like terms*

675 = 15x

*Divide by 15*

x = 45