recall that to multiply a vector by a scalar quantity we just multiply each component by the scalar, so

2a = 2<7,4> = <2*7, 2*4> = <14,8>

also recall, that to add or subtract two vectors, we add or subtract the corresponding components:

so 2a - b = 2<7,4> - <2,3> = <14,8> - <2,3> = <(14 - 2), (8 - 3)> = <12,5>