First we need to make the fractions "compatible" for adding them easily, we can do that by equating their denominators using the common minimum multiple of their denominators 3 and 5 which is 15, so we have to multiply and divide to not modify them for a correct number, lets show that up:
1/3 = (5/5)(1/3) = 5*1/5*3 = 5/15, therefore: 1/3 = 5/15
4/5 = (3/3)(4/5) = 3*4/3*5 = 12/15, therefore 4/5 = 12/15
now we do the sum first:
1/3 + 4/5
= 5/15 + 12/15
= 17/15
and finally we multiply by 7
7*(17/15) = 119/15
