Question

What the hell am I doing wrong here ?

Answer 1

No idea what you're doing wrong specifically.  But:

If we roast Xg of pork we'll only end up with 0.79Xg of useable meat.

We need 190\*299g of useable meat.

So we need 0.79X = 190\*299

Solve for X

Answer 2

How are we supposed to say what you did wrong if we can't see what you did at all?  I've tried a few different seemingly plausible mistakes and still cannot replicate your answer.