Using the Consequence Rule I have the following example that I have to prove

{a&gt;7 ^ b&gt;=0} n:=a-b {n&lt;a ^ a+b&gt;=0}


Using the Consequence Rule I assumed that the P is true

{true} n=a-b {n&lt;a ^ a+b&gt;=0}


so logically, the Q should be true, but I'm not sure if that'd be enough.
Here is what you need to prove:


  If $a &gt; 7$, $b \geq 0$ and $n = a-b$ then $n &lt; a$ and $a+b \geq 0$.


By the way, this is actually false, since if $b = 0$ then $n = a$. Perhaps you miscopied?