8 /21 = - 2/7x.
Cross multiply by multiplying both the numerator of one fraction with the denominator of the other fraction., equating it to the other numerator multiplied by the other denominator.
7x*8 = -2*21
x*7*8 = -2*21
x = -(2*21) / (7*8). 2 into 8 is 4. 7 into 21 is 3.
x = - 3/4
I hope this helps.