Add brackets to make the intended meaning clear. Is this:

(-7/20) x = -21/10

if so multiply both sides by -10/7 to get:

(-10/7)(-7/20) x = (-10/7)(-21/10)

which is:

(1/2)x = 3

or x=6

This needs to be made clearer, I have no idea what your question is.(4.9-18.6)x 13

RonL