Ax + by=c as slope intercept form

Saw the image.

Given: 
Whole wall is 13 2/3 ft by 25ft
4 menus (gray areas) with measurements of 6 ft wide and 7 1/2 ft tall.

Convert mixed fractions into improper fractions.

13 2/3 ft = ((13*3)+2)/3 = 41/3

Area of the Wall = 41/3 ft * 25 ft = (41*25)/3 = 1025/3 ft² = 341 2/3 ft²

7 1/2 ft = ((7*2)+1)/2 = 15/2

Area of the menu = 15/2 ft * 6 ft = (15*6)/2 = 90/2 = 45 ft²
45 ft² x 4 menus = 180 ft² 

Area of wall space not covered by the chalkboard paint:
A = 341 2/3 ft² - 180 ft² = 161 2/3 ft²