The right triangle shown has angles x, y, and z and side lengths a, b, and c. which equation is not true for this triangle?

So attached is a picture of the triangle you are talking about and listed under are the choices:

A.) Cos Z=b/c 
B.) Sin X=c/b 
C.) Tan X=b/a 
D.) Tan Z=a/b

The answer would then be B. SinX = c/b.

Just remember SOH CAH TOA:

Sinθ=  Opposite         Cosθ =    Adjacent             Tanθ=   Opposite
         Hypotenuse                   Hypotenuse                        Adjacent

Using the triangle in the scenario, you just need to identify which side is which. 

Given m∠Z
Adjacent = b
Opposite = a
Hypotenuse = c

SinZ=  a         CosZ = b             TanZ= a 
            c                      c                        b

Given m∠X:

Adjacent = b
Opposite = a
Hypotenuse = c

SinX=  b         CosX = a             TanX= b 
            c                      c                        a

So the answer is B. 

Attached is a picture of how I assigned the sides depending on the angle used.