Two bricks, of mass m1 and m2, are separated by a distance r. the gravitational force of attraction between them is 1.5x10-8 n. what is the gravitational force of attraction if: a) the distance between them is doubled? b) one brick is replaced by another brick that weights twice as much as one of the original bricks?

I'm not going to check on the number in the question.
I'll assume the gravitational force between them is really 1.5 x 10⁻⁸  N.

a).  The gravitational force between two objects is inversely proportional
to the distance between them.  So if the distance between them is doubled,
the gravitational force falls to  1/4  of what it was originally. 
That's   3.75 x 10⁻⁹ N .

b).  The gravitational force between two objects is directly proportional
to the product of their masses.  So if you double the mass of one of the
objects, the product of their masses also doubles, and so does the
gravitational force. 
So it's now  3 x 10⁻⁸ N .