Two point charges of equal magnitude are
7.5 cm apart. at the midpoint of the line
connecting them, their combined electric field
has magnitude of 45 n/c. find the magnitude
of the charges.

Q 1 = q 2 = Q
E = k * Q / r²
The combined electric field:
E = 2 * k * Q / r²
k = 9 · 10^9 Nm²/C²
r = 0.075 : 2 = 0.0375 m
45 N/C = 2 · 9 · 10^9 · Q /  0.0375²
45 = 18 · 10 ^9 Q / 0.0014062
Q = 45 · 0.0014062 / 18 · 10^9
Q = 0.003515 · 10^(-9) C 
Q = 3.5155 · 10 ^ (-12) C = 3.5155 pC