This question can be answered in two ways: graphically and geometrically, or by using vectors.
GEOMETRIC
The vectors are points (6,4) and (6,2), that is, the coefficients of the unit vectors i and j are the x and y coords respectively.
The angle θ is the difference between the angles of the slopes of the red and blue lines. The red line has a slope of 4/6=2/3 and the blue line has a slope of 2/6=1/3. The slopes are tangents of two angles, and θ=arctan(2/3)-arctan(1/3)=33.69°-18.43°=15.26° approx (0.266 radians).
VECTORS
The dot-product v.w=|v||w|cosθ.
v.w=6×6+2×4=36+8=44.
|v|=√(6²+2²)=√40, |w|=√(6²+4²)=√52.
cosθ=44/√2080=0.9648, θ=arccos(0.9648)=15.26°.