veca+vecb is one diagonal of the parallelogram.
veca-vecb is the other diagonal of the parallelogram.
`cos(theta)=(vecx*vecy)/(|vecx||vecy|)`
`vecx=veca+vecb=(4,5)`
`vecy=veca-vecb=(6,-3)`
`vecx*vecy=4*6+5*(-3)=24-15=9`
`|vecx|=sqrt(4^2+5^2)=sqrt(16+25)=sqrt(41)`
`|vecy|=sqrt(6^2+(-3)^2)=sqrt(36+9)=sqrt(45)`
`cos(theta)=(9)/(sqrt(41)sqrt(45))=9/(sqrt(41)(3sqrt(5)))`
`theta=cos^(-1)(3/(sqrt(41)(sqrt(5))))=77.9^o` our answer