The first equation can be written: 3r=4s, so we can write r=4s/3 by dividing through by 3. Then we substitute this vale in the other equation: 8s/3+5s=23. We get rid of the fraction by multiplying through by 3: 8s+15s=69; 23s=69; s=3, by dividing through by 23. now we can find r=4s/3=4*3/3=4. To see if that's correct, substitute for r and s in the original equations: 3*4-4*3=12-12=0; 2*4+5*3=8+15=23. Correct!