First, find sin(a) and sin(b)...

if cos(a) = 2/5 then sin(a) = +/- sqrt(1 - 4/25) = sqrt(21)/5 (it's in qII)

if cos(b) = 1/5 then sin(b) = +/- sqrt(1 - 1/25) = sqrt(24)/5 = 4sqrt(6)/5 (in qII)

so

sin(a+b)

= sin(a)cos(b) + sin(b) cos(a)

= sqrt(21)/25 + 8sqrt(6)/25

= (sqrt(21) + 8sqrt(6))/25

and

sin(a-b)

= sin(a)cos(b) - sin(b)cos(a)

= (sqrt(21) - 8sqrt(6))/25

Google it