$\sqrt{ 49 \times \left( -1 \right) }$
The root of a product is equal to the product of the roots of each factor$\sqrt{ 49 }\sqrt{ -1 }$
Write the number in exponential form with the base of $7$$\sqrt{ {7}^{2} }\sqrt{ -1 }$
Use $\sqrt[2]{-1}=i$ to simplify the expression$\sqrt{ {7}^{2} }i$
Reduce the index of the radical and exponent with $2$$7i$