Decrypting an Affine Cipher with known characters
You have 3 equations and 2 unknowns, so it is solvable, assuming a solution exists. You can plug this into any linear equation solver.

If you subtract equation 3 from equation 2, you get $a=-3$, and can solve for $b=75$. This fits equation 2 and 3, but not equation 1. So, no solution exists.