I need it fast, I'm timed

It is B

Step-by-step explanation:

Plug each value possible into the equation

(48+x) / 31   =    x/15

If both sides are equal when you plug x in, it's correct

If they aren't equal, it is wrong.

How did I get this equation?

Lets use proportions to solve.

Since they are similar, (the two triangles are similar, meaning that they have the same ratio of the y and x axis values) you can use proportions meaning you make it so x over 15 is equal to the total length by the total bottom length.

If you plug 45 in as x, you get (48 + 45) divided by 31 is equal to 45/15

If you simplify the equation, you get 93 divided by 31 is equal to 3, of which 93 divided by 31 is 3

I think you know that 3=3

so it is correct.