using the standard formula for a OLS slope coefficient:
r=0.5 or -0.5
interesting problem. I agree with you that you should choose the negative solution since both coefficients are negative.
I have a small question please:
An experiment was made to find correlation between X and Y.
2 Regression lines has been build:
It is known that: m=-0.4 and b=-0.625
Find the correlation r.
Is the answer -0.5 ?