Answer is at 900 miles the costs will be equal for both rental companies.
Step by step
Using slope intercept formula
y = mx + b
m = rate or slope, here it is the price per mile
x = the variable, the unknown miles
b = the flat rate or starting set cost
a+ rental
y = .10x + 175
zippy rental
y = .05 + 220
Make the two equations equal each other to solve
.10x + 175 = .05x + 220
Subtract .05x from each side to isolate variable
.10x - .05x + 175 = .05x - .05x + 220
Simplify
.05x + 175 = 220
Now subtract 175 from both sides to isolate constants
.05x + 175 - 175 = 220 - 175
Simplify
.05x = 45
Divide both sides by .05 to solve for x
.05/.05x = 45/.05
x = 900
So at 900 miles driven the costs will be the same.