The efficiency \( \eta \) of a Carnot engine is given by the following equation:
\[
\eta = 1 - \frac{T_{{sink}}}{T_{{source}}}
\]
where:
- \( \eta \) is the efficiency of the engine,
- \( T_{{sink}} \) is the temperature of the sink (cold reservoir),
- \( T_{{source}} \) is the temperature of the source (hot reservoir).
We are given:
- \( \eta = 50% = 0.5 \),
- \( T_{{sink}} = 350 \, {K} \).
Substitute these values into the efficiency equation:
\[
0.5 = 1 - \frac{350}{T_{{source}}}
\]
Solving for \( T_{{source}} \):
\[
\frac{350}{T_{{source}}} = 1 - 0.5 = 0.5
\]
\[
T_{{source}} = \frac{350}{0.5} = 700 \, {K}
\]
Thus, the temperature of the source is \( 700 \, {K} \).
\[
\boxed{700 \, {K}}
\]