Find out the relation between acceleration a and b as shown in following figure

Explanation

Mark the points on the string which is attached to the real object (e.f,g,h)

Step 2. Acceleration of each point which are responsible to effect the length of string `a_e = 0` (because it is attached to fixed object)
`a_f = -b` (attach to pulley which is moving with wedge's acceleration & –ve because it decreases the length)
`a_g = b cos theta` (only this component is responsible to effect the length of string) `a_h = (a – b cos theta)` (resultant velocity at point h along the string)
So now from `3^(rd)` format `a_e + a_f + a_g + a_h = 0`
`implies 0 + (- b) + b cos theta + (a – b cos theta) = 0`
`a – b = 0`
`implies a = b`.