I can not figure out this problem and it is starting to annoy me!

So far I have got (may be wrong)

1.

L encrypted as J implies 12 a + b ≅ 10 mod 26

G encrypted as C implies 7a + b ≅ 3 mod 26

my set of notes then say to subtract the first congruence from the 2nd which gave me:

-5 a ≅ -7 mod 26

keep getting mixed up after here when trying to find the value of a and b

any help would be greatly appreciated

question is as follows: