Simultaneous equations with a modulus

Hi, Im have trouble figuring out how to solve simultaneous equations of the form:

4a + b (mod 26) = 20

19a + b (mod 26) = 25

I know how to do simultaneous equations normally but with the modulus I keep messing up somewhere.

I multiplied everything in the top equation by + 1 and everything in the bottom equation by -1 to get:

4a + b (mod 26) = 20

-19a - b (mod 26) = -25

then add them to get:

-15a = -5

It seems obvious to me that Im doing something wrong but I've had trouble finding out how to do this properly. Thank you.