pH=-log (1.2*10^3)

Use PEMDAS to solve for your pH.

the parentheses holding 1.2*10^3 show that you should do that computation first.

So, using PEMDAS inside these parentheses says you should do the Exponent computation first.

$${{\mathtt{10}}}^{{\mathtt{3}}} = {\mathtt{1\,000}}$$

Exponents can be written alternatively as multiplying your base by itself so many times, in this case, 10 is the base and the exponent is 3 so you would multiply 10 by itself 3 times.

$${\mathtt{10}}{\mathtt{\,\times\,}}{\mathtt{10}}{\mathtt{\,\times\,}}{\mathtt{10}} = {\mathtt{1\,000}}$$

So, then this becomes the number inside your parentheses.

$${\mathtt{1.2}}{\mathtt{\,\times\,}}{{\mathtt{10}}}^{{\mathtt{3}}} = {\mathtt{1\,200}}$$

Written alternatively as,

$${\mathtt{1.2}}{\mathtt{\,\times\,}}{\mathtt{1\,000}} = {\mathtt{1\,200}}$$

Then the next step would be to preform the logarithm of the newfound number,

$${log}_{10}\left({\mathtt{2\,100}}\right) = {\mathtt{3.322\: \!219\: \!294\: \!733\: \!919}}$$

Then take the negative of this number, or multiply by -1,

$${\mathtt{\,-\,}}{log}_{10}\left({\mathtt{2\,100}}\right) = -{\mathtt{3.322\: \!219\: \!294\: \!733\: \!919}}$$

Typically, pH is shorted to the first two decimal places, and it has no units, other than putting pH after the number. So that means your answer is

$$-{\mathtt{3.22}}{pH}$$

You have a negative number for your pH because pH is typically used for VERY small numbers, as it calculates the amount of or concentration of Hydrogen present in the solution.

Hope this helps!