Substituting x=-4 and y=0 gives: -4k+3=0,

∴k=，

The analytical formula of the straight line is y=x+3,

② It is known that the coordinate of point P is (1, m),

∴m=× 1+3=；

(2)∫PP′∨AC，

△PP′D∽△ACD，

= =, that is =,

∴a=；

(3) When the point p is in the first quadrant,

1) If ∠ AP ′ c = 90, P ′ a = P ′ c (as shown in figure 1).

Passing point p' is the axis of P'H⊥x at point H.

∴pp′=ch=ah=p′h=ac.

∴2a=(a+4)，

∴a=，

2) If ∠ p ′ ac = 90 and p ′ a = c,

Then PP'= AC,

∴2a=a+4，

∴a=4，

3) If ∠ p ′ ca = 90,

Then points p' and p are in the first quadrant, which contradicts the condition.

∴△ p ′ ca cannot be an isosceles right triangle with C as the right vertex.

∴a All values that meet the conditions are a=4 or.