(a) First factorise and cancel where possible

4x+1/3x-5 + 2x/12x-20 = 4x+1/3x-5 + 2x/4(3x-5) = 4x+1/3x-5 + x/2(3x-5)

Then multiply the top and bottom of each fraction so that they have the same denominator

= 2(4x+1)/2(3x-5) + 2x/4(3x-5)

Then add the fractions together

= (8x+2+2x)/2(3x-5) = (10x+2)/2(3x-5)

Finally factorise and cancel down if possible

= 2(5x+1)/2(3x-5) = (5x+1)/(3x-5)

(b) similar