Running this in my debugger showed that you're getting floating point representation errors. This means that although technically you should be getting numbers perfectly rounded to 1 decimal given that you're applying increments of 0.1, in reality this isn't the case:

As you can see, r = -2.0 and i = -2.00...4, thus at no point is r == i.

You can fix this by adding another round statement at the end:

print("enter the first m: ")
m = input()  # m = slope

print("enter the first b: ")
b = input()  # b = y-intercept

print("enter the second m: ")
m1 = input()

print("enter the second b: ")
b1 = input()

sub_m = int(m) - int(m1) #sub = subtract
sub_b = int(b) - int(b1)

if (sub_m == 0):
    print("parallel")

x = float(-sub_b / sub_m)
r = round(x, 1)

i = 0.0
while i != r:

    print(r, i)

    if (sub_m > 0 and sub_b > 0):
        i -= 0.1
    elif (sub_m < 0 and sub_b < 0):
        i -= 0.1
    else:
        i += 0.1
    i = round(i, 1)  # <- this

print(f"Hit pt: {i}")

HOWEVER: This is still error prone, and I recommend finding a way to avoid if i==r altogether in the code. If i is lower than r, exit the loop when it finally becomes bigger, and viceversa. Its best practice to avoid using the == condition when comparing floats, and to find a way to use <= and >=.

You could use exceptions. But you only should use exceptions for stuff that isn't supposed to happen. So not for this.

That is why I recommand signals:

import sys, signal
def signal_handler(signal, frame):
    print("\nprogram exiting gracefully")
    sys.exit(0)

signal.signal(signal.SIGINT, signal_handler)

you should put this on the beginning of your program and when you press ctrl+c wherever in your program it will shut down gracefully

Code explanation:

You import sys and signals. Then you make a function that executes on exit. sys.exit(0) stops the programming with exit code 0 (the code that says, everything went good).

When the program get the SIGINT either by ctrl-c or by a kill command in the terminal you program will shutdown gracefully.