Answer:
The mural is 150 inches long
Step-by-step explanation:
We can turn this given measurements in the problem into porportions to find out the new wide that we need like this:
The relationship between the wide of the photograph and the wide of the mural can be written in a fraction like this:

The relationship between the length of the photograph and the length of the mural can be written in a fraction like this:

where x is the length of the mural.
both ratios are the same because we maintian the proportion so we can put them like this:

we solve for 

The mural is 150 inches long