I am checking to see if this is done correctly, the slot is a little confusing for me.
B is a pin in a slot, solve for the spring constant (k) so the 50 lb force applied does not move pin B
Solving for the moment about A I get
50lbs*2" - sin45(N)*3"
N = 47.1 lbs
Then solving for forces in the Y direction I get
k*.125" - sin45(N) ==> k = 33.2lbs/(.125") = 266 lbs/in