The electric field of an electromagnetic wave in free space is
$\overrightarrow{\mathrm{E}}=57\cos \left[7.5\times {10}^6\mathrm{t}-5\times {10}^{-3}(3\mathrm{x}+4\mathrm{y})\right](4\widehat{\mathrm{i}}-3\widehat{\mathrm{j}})\mathrm{N}/\mathrm{C}$
The associated magnetic field in Tesla is-

$\overrightarrow{\mathrm{B}}=\frac{57}{3\times {10}^8}\cos \left[7.5\times {10}^6\mathrm{t}-5\times {10}^{-3}(3\mathrm{x}+4\mathrm{y})\right]\left(\widehat{\mathrm{k}}\right)$

$\overrightarrow{\mathrm{B}}=\frac{57}{3\times {10}^8}\cos \left[7.5\times {10}^6\mathrm{t}-5\times {10}^{-3}(3\mathrm{x}+4\mathrm{y})\right]\left(5\widehat{\mathrm{k}}\right)$

$\overrightarrow{\mathrm{B}}=-\frac{57}{3\times {10}^8}\cos \left[7.5\times {10}^6\mathrm{t}-5\times {10}^{-3}(3\mathrm{x}+4\mathrm{y})\right]\left(\widehat{\mathrm{k}}\right)$

$\overrightarrow{\mathrm{B}}=-\frac{57}{3\times {10}^8}\cos \left[7.5\times {10}^6\mathrm{t}-5\times {10}^{-3}(3\mathrm{x}+4\mathrm{y})\right]\left(5\widehat{\mathrm{k}}\right)$