y = ax + b 
a = he so goc ≠ 0, 
do thi ham so la duong thang, cat truc tung tai B(0,b).
a) Goi (d): y = ax + b
he so goc = 3 => a = 3 => y = 3x + b
(1, 2) ∈ d => 2 = 3*1 + b => b = -1
Vay (d): y = 3x - 1
b) (d): y = ax + b,  (d1): y = 1* x - 2
+  d // d1 => hai duong thang d va d1 co cung he so goc va cat truc tung tai 2 diem khac nhau.
=> a = 1,  b ≠ -2
+ d cat truc tung tai B(0, b) voi tung do bang 5
=> b = 5. (thoa man b ≠ -2)
Vay (d): y = x + 5