333

\overline{u}=\overline{\text{i}}+r\overline{\text{j}}
\overline{v}=7\overline{\text{i}}+4\overline{\text{j}}
\overline{u}\cdot\overline{v}=7\cdot1+4\cdot r=0
4r=-7\ \parallel:4
r=-\frac{7}{4}